Rear focusing zoom lens

ABSTRACT

A zoom lens having a first lens unit remaining stationary and having positive refracting power, a second lens unit having negative refracting power, a third lens unit having positive refracting power, a fourth lens unit having negative refracting power and a fifth lens unit remaining stationary and having positive refracting power to perform zooming by moving at least the second lens unit and the fourth lens unit and to perform focusing an infinite object to a nearest object by moving the fourth lens unit toward the image surface.

This application is a division of Application No. 08/485,136 filed Jun.7, 1995 now U.S. Pat. No. 5,847,882.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a rear focusing zoom lens, and, moreparticularly, to a rear focusing zoom lens for use in a photographingcamera, a video camera, a broadcasting camera or the like and exhibitinga high power-variable ratio and aperture ratio, relatively wide angle ofview and short overall length such that the variable power ratio is 6 to20 and the F-number is about 1.6 to 1.8 at a wide angle end thereof.

2. Related Background Art

Recently, the trend of reducing the size and weight of a personal videocamera or the like has caused the sizes of photographing lenses to bereduced significantly. In particular, the overall length of the lens andthe diameter of the front lens have been intended to be decreased, andthe structure has been desired to be simplified.

As a means for achieving the foregoing objects, a so-called rearfocusing zoom lens has been known, in which focusing is performed bymoving lens units other than the first lens unit near an object.

Generally, in the rear focusing zoom lens, the first group has a smalleffective aperture as compared with that of a zoom lens of a type inwhich focusing is performed by moving the first lens unit. Therefore,the size of the overall lens system can easily be reduced, and close-upphotographing, in particular, very close-up photographing can easily beperformed. Furthermore, since relatively small and light lens units aremoved to perform the close-up photographing operation or the veryclose-up photographing operation, the power required to move the lensunits can be reduced, and, therefore, focusing of an image can quicklybe performed.

A rear focusing zoom lens of the foregoing type has been disclosed inJapanese Patent Laid-Open No. 5-215967, which comprises five lens unitsconsisting of a first lens unit having positive refracting power that isfixed during the power variation, a second lens unit having negativerefracting power for varying the power, a third lens unit havingpositive refracting power, a fourth lens unit having negative refractingpower, and a fifth lens unit having positive refracting power forcorrecting change in the image surface occurring when the power isvaried, the lens units being disposed in the foregoing sequential orderwhen viewed from the object. When the power is varied, the third and/orthe fourth lens unit is moved. When focusing is performed, at least anyof the third, fourth and the fifth lens unit is moved.

The applicant of the present invention has disclosed a rear focusingzoom lens in Japanese Patent Laid-Open No. 6-148523 which comprises fivelens units consisting of a first lens unit having positive refractingpower, a second lens unit having negative refracting power, an aperturestop, a third lens unit having positive refracting power, a fourth lensunit having negative refracting power, and a fifth lens unit havingpositive refracting power, the lens units being disposed in theforegoing sequential order when viewed from the object. The second lensunit is moved toward the image surface to vary the power from the wideangle end to the telescopic end. Change in the image surface occurringwhen the power is varied is corrected by moving the third and fourthlens units. Focusing is performed by moving at least any of the second,third and fourth lens units along the optical axis.

In general, if a zoom lens employs the rear focusing method, the size ofthe overall lens system can be reduced, focusing can be completedfurther quickly and further close-up photographing can be performed.

On the contrary, excessive aberration change occurs at the time ofperforming focusing, and there arises a problem in that a difficultyarises in reducing the size of the overall lens system from an infiniteobject to a near object while obtaining excellent optical performance.

If the zooming method is employed in which the three lens units aremoved when the power is varied as disclosed above, the zooming mechanismwill be undesirably complicated and the size of the lens mount cannot bereduced.

The zooming method, as disclosed in Japanese Patent Laid-Open No.5-215967, in which the fifth lens unit, located near the image surfacewhen the power is varied, is moved, causes the mechanism of thesolid-state image sensing device, such as a CCD, and that near the filmto be complicated. If dust adheres to the lens surface adjacent to theimage surface, the dust image is undesirably taken on the film or thelike and the image quality deteriorates.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a smallzoom lens exhibiting excellent power variable ratio.

Another object of the present invention is to provide a rear focusingzoom lens which is capable of performing rapid focusing.

Another object of the present invention is to provide a rear focusingzoom lens having excellent optical performance.

According to one aspect of the present invention, there is provided arear focusing zoom lens comprising: a first lens unit remainingstationary and having positive refracting power; a second lens unithaving negative refracting power; a third lens unit having positiverefracting power; a fourth lens unit having negative refracting power;and a fifth lens unit remaining stationary and having positiverefracting power, which are disposed in this sequential order whenviewed from an object, wherein at least the second lens unit and thefourth lens unit are moved to perform zooming, and the fourth lens unitis moved toward an image surface to perform focusing of an infiniteobject to a nearest object.

Furthermore, the fourth lens unit is moved to form a concave locusfacing the object when the fourth lens unit is moved from the wide angleend to the telescopic end to save the space and to reduce the diameterof the first lens unit. Furthermore, various optical conditions aresatisfied so that focusing is performed satisfactorily and excellentoptical performance is obtained.

Other and further objects, features and advantages of the invention willbe evident from the following detailed description of the preferredembodiments in conjunction with the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing paraxial refracting power configuration of azoom lens according to the present invention;

FIG. 2 is a cross sectional view of a lens according to Example 1 of thepresent invention at an end;

FIG. 3 is a cross sectional view of a lens according to Example 2 of thepresent invention at a wide angle end;

FIG. 4 is a cross sectional view of a lens according to Example 3 of thepresent invention at a wide angle end;

FIG. 5 is a cross sectional view of a lens according to Example 4 of thepresent invention at a wide angle end;

FIG. 6 is a cross sectional view of a lens according to Example 5 of thepresent invention at a wide angle end;

FIG. 7 is a cross sectional view of a lens according to Example 6 of thepresent invention at a wide angle end;

FIG. 8 is a cross sectional view of a lens according to Example 7 of thepresent invention at a wide angle end;

FIG. 9 is a cross sectional view of a lens according to Example 8 of thepresent invention at a wide angle end;

FIG. 10 is a cross sectional view of a lens according to Example 9 ofthe present invention at a wide angle end;

FIG. 11 is a cross sectional view of a lens according to Example 10 ofthe present invention at a wide angle end;

FIG. 12 is a cross sectional view of a lens according to Example 11 ofthe present invention at a wide angle end;

FIG. 13 is a cross sectional view of a lens according to Example 12 ofthe present invention at a wide angle end;

FIG. 14 is a cross sectional view of a lens according to Example 13 ofthe present invention at a wide angle end;

FIGS. 15 (A)(1)-15(A)(4) and 15(B)(1)-15(B)(4) are graphs showingvarious aberrations according to Example 1 of the present invention;

FIGS. 16 (A)(1)-16(A)(4) and 16 (B)(1)-16(B)(4) are graphs showingvarious aberrations according to Example 2 of the present invention;

FIGS. 17(A)(1)-17(A)(4) and 17(B)(1)-17(B)(4) are graphs showing variousaberrations according to Example 3 of the present invention;

FIGS. 18(A)(1)-18(A)(4) and 18(B)(1)-18(B)(4) are graphs showing variousaberrations according to Example 4 of the present invention;

FIGS. 19(A)(1)-19(A)(4) and 19 (B)(1)-19(B)(4) are graphs showingvarious aberrations according to Example 5 of the present invention;

FIGS. 20(A)(1)-20(A)(4) and 20 (B)(1)-20(B)(4) are graphs showingvarious aberrations according to Example 6 of the present invention;

FIGS. 21 (A)(1)-21(A)(4) and 21(B)(1)-21(B)(4) are graphs showingvarious aberrations according to Example 7 of the present invention;

FIGS. 22(A)(1)-22(A)(4) and 22(B)(1)-22(B)(4) are graphs showing variousaberrations according to Example 8 of the present invention;

FIGS. 23(A)(1)-23(A)(4) and 23(B)(1)-23(B)(4) are graphs showing variousaberrations according to Example 9 of the present invention;

FIGS. 24(A)(1)-24(A)(4) and 24(B)(1)-24(B)(4) are graphs showing variousaberrations according to Example 10 of the present invention;

FIGS. 25(A)(1)-25(A)(4) and 25(B)(1)-25(B)(4) are graphs showing variousaberrations according to Example 11 of the present invention;

FIGS. 26(A)(1)-26(A)(4) and 26(B)(1)-26(B)(4) are graphs showing variousaberrations according to Example 12 of the present invention;

FIGS. 27(A)(1)-27(A)(4) and 27(B)(1)-27(B)(4) are graphs showing variousaberrations according to Example 13 of the present invention;

FIG. 28 is a cross sectional view of a lens according to Example 14 ofthe present invention;

FIG. 29 is a cross sectional view of a lens according to Example 15 ofthe present invention;

FIG. 30 is a cross sectional view of a lens according to Example 16 ofthe present invention;

FIG. 31 is a cross sectional view of a lens according to Example 17 ofthe present invention;

FIG. 32 is a cross sectional view of a lens according to Example 18 ofthe present invention;

FIG. 33 is a cross sectional view of a lens according to Example 19 ofthe present invention;

FIGS. 34(A)(1)-34(A)(4) and 34(B)(1)-34(B)(4) are graphs showing variousaberrations according to Example 14 of the present invention;

FIGS. 35(A)(1)-35(A)(4) and 35(B)(1)-35(B)(4) are graphs showing variousaberrations according to Example 15 of the present invention;

FIGS. 36(A)(1)-36(A)(4) and 36(B)(1)-36(B)(4) are graphs showing variousaberrations according to Example 16 of the present invention;

FIGS. 37(A)(1)-37(A)(4) and 37(B)(1)-37(B)(4) are graphs showing variousaberrations according to Example 17 of the present invention;

FIGS. 38(A)(1)-38(A)(4) and 38(B)(1)-38(B)(4) are graphs showing variousaberrations according to Example 18 of the present invention;

FIGS. 39(A)(1)-39(A)(4) and 39(B)(1)-39(B)(4) are graphs showing variousaberrations according to Example 19 of the present invention;

FIG. 40 is a cross sectional view of a lens according to Example 20 ofthe present invention at the wide angle end;

FIG. 41 is a cross sectional view of a lens according to Example 21 ofthe present invention at the wide angle end;

FIG. 42 is a cross sectional view of a lens according to Example 22 ofthe present invention at the wide angle end;

FIG. 43 is a cross sectional view of a lens according to Example 23 ofthe present invention at the wide angle end;

FIG. 44 is a cross sectional view of a lens according to Example 24 ofthe present invention at the wide angle end;

FIG. 45 is a cross sectional view of a lens according to Example 25 ofthe present invention at the wide angle end;

FIG. 46 is a cross sectional view of a lens according to Example 26 ofthe present invention at the wide angle end;

FIG. 47 is a cross sectional view of a lens according to Example 27 ofthe present invention at the wide angle end;

FIGS. 48(A)(1)-48(A)(4) and 48(B)(1)-48(B)(4) are graphs showing variousaberrations according to Example 20 of the present invention;

FIGS. 49(A)(1)-49(A)(4) and 49(B)(1)-49(B)(4) are graphs showing variousaberrations according to Example 21 of the present invention;

FIGS. 50(A)(1)-50(A)(4) and 50(B)(1)-50(B)(4) are graphs showing variousaberrations according to Example 22 of the present invention;

FIGS. 51(A)(1)-51(A)(4) and 51(B)(1)-51(B)(4) are graphs showing variousaberrations according to Example 23 of the present invention;

FIGS. 52(A)(1)-52(A)(4) and 52(B)(1)-52(B)(4) are graphs showing variousaberrations according to Example 24 of the present invention;

FIGS. 53(A)(1)-53(A)(4) and 53(B)(1)-53(B)(4) are graphs showing variousaberrations according to Example 25 of the present invention;

FIGS. 54(A)(1)-54(A)(4) and 54(B)(1)-54(B)(4) are graphs showing variousaberrations according to Example 26 of the present invention; and

FIGS. 55(A) (1)-55(A) (4) and 55(B) (1)-55(B) (4) are graphs showingvarious aberrations according to Example 27 of the present invention;

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a diagram showing paraxial refracting power configuration of azoom lens according to the present invention, which is the essentialportion of the present invention. FIGS. 2 to 14 show the cross sectionof lenses according to this embodiment, and FIGS. 15(A)(1) to 27(B)(4)show the aberrations of each lens. In each aberration graph, graph (A)shows each wide angle end and (B) shows each telescopic end.

Referring to FIG. 1, reference numeral I represents a first lens unithaving positive refracting power, II represents a second lens unithaving negative refracting power, III represents a third lens unithaving positive refracting power, IV represents a fourth lens unithaving negative refracting power, and V represents a fifth lens unithaving positive refracting power. Symbol IM represents an image surfaceat which a solid-image sensing device, such as a CCD, is disposed.Symbol SP represents an aperture. Zooming is performed from the wideangle end to the telescopic end in directions as indicated by arrows,that is, the second lens unit is moved toward the image surface and thefourth lens unit is moved toward the object to form a concave locus.Note that the continuous line indicates the locus of movement withrespect to an infinite object and a dashed line indicates the locus withrespect to a near object. Thus, focusing from an infinite object to anear object is performed by moving the fourth lens unit IV toward theimage. Note that at least the first and fifth lens units always remainstationary during the zooming operation.

In place of the conventional focusing method in which the first lensunit is moved forwards, the fourth lens unit is moved to performfocusing so as to reduce the diameter of the front lens and to permitfocusing of a near object to be performed.

Furthermore, the zoom lens according to this embodiment has a structuresuch that the second lens unit is moved in such a manner that thehorizontal magnification β₂ crosses the equal magnification (β₂ =-1)from the wide angle end to the telescopic end so as to obtain a highvariable power ratio. Furthermore, change in the image surface occurringwhen the power is varied is, as described above, corrected by moving thefourth lens unit toward the object to form a concave locus facing theobject. As a result, the space efficiency can be improved, and thediameter of the first lens unit can be shortened. Furthermore, a highzooming ratio (about 20 times in a major portions of examples) can beobtained.

Although it is preferable that the third lens unit be fixed to simplifythe structure of the lens mount, the third lens unit may be moved tocause the third lens unit to share the equal magnification. Thus, thesize reduction and high zooming ratio can simultaneously be realized.

As a result of employment of the foregoing structure, a zoom lens havinga high zooming ratio and a small size can be provided. Furthermore, itis preferable that the following condition be satisfied.

Assuming that the focal length of the overall system at the wide angleend of the system and the distance from the fourth lens unit to thefifth lens unit with respect to an infinite object are F_(W) and D_(4W),respectively, the following relationship is held:

    1.80<D.sub.4W /F.sub.W <3.9

The foregoing expression provides the range for correcting change in theimage surface occurring due to the zooming, and the range, in which thefourth lens unit for performing focusing is moved. If the value issmaller than the lower limit, focusing of a sufficiently near objectcannot easily be performed. If the value is larger than the upper limit,the overall size of the lens is enlarged excessively though the focusingcan be performed easily.

The rear focusing zoom lens has a characteristic that the amount of theforward movement of the fourth lens unit, which is the focusing lens, isenlarged near the wide angle end than that near the telescopic end. Theforegoing tendency becomes apparent in proportion to the zooming ratio.As contrasted with this, this embodiment has a structure as follows:assuming that the distance from the second lens unit to the third lensunit at the wide angle end of the system with respect to an infiniteobject is D_(2W) and the distance from the fourth lens unit to the fifthlens unit at the wide angle end of the system with respect to aninfinite object is D_(4W), the following relationship is held:

    1.5<D.sub.2W /D.sub.4W <3.5

Thus, a desired zooming ratio can be obtained and excellent focusing canbe performed.

D_(2W) is a range, in which the second lens unit serving as a main lensfor varying the power is moved, and D_(4W) is a range, in which thefourth lens unit for correcting change in the image surface andperforming focusing operation is moved. By providing the range in whicheach lens unit is moved to satisfy the foregoing condition, appropriatezooming magnification and close distance can easily be realized.Although a zooming ratio can easily be increased if the value is largerthan the upper limit, a difficulty arises in providing an amount inwhich the fourth lens unit is moved at the time of performing focusing.If the value is smaller than the lower limit, a desired zooming ratiocannot be maintained.

It is preferable that the following conditions be satisfied in relationto the optical power of the fourth lens unit:

    0.4<|D.sub.4W /F.sub.4 |<1.0

where F₄ is the focal length of the fourth lens unit. If the value issmaller than the lower limit, a difficulty arises in maintaining anamount of movement for the purpose of performing focusing. As a result,focusing of a near object cannot easily be performed. If the value islarger than the upper limit, the size of the overall lens system cannotbe reduced though the focusing can be performed easily.

As described above, the amount of focusing at the short distance islarge at the telescopic end. It is preferable that a zoom lens of a typecapable of considerably varying the power meets the following range:

    0.63<|D.sub.4W /F.sub.4 |<1.0

Although a structure relative to the fourth lens unit has beendescribed, satisfaction of the following condition will reduce thediameter of the front lens, realize a high zooming ratio andsatisfactorily correct the aberration, in particular, the curvature offield.

That is, assuming that the focal lengths of the first lens unit and thesecond lens unit are F₁ and F₂, respectively, the following condition isrequired to be satisfied:

    5.7<|F.sub.1 /F.sub.2 |<7

If the value is smaller than the lower limit, the considerable powervariation raises a necessity of enlarging the amount of movement of thesecond lens unit. Thus, the size cannot be reduced and the diameter ofthe front lens is enlarged excessively. If the value is larger than theupper limit, the Petzval sum is enlarged excessively in a range ofnegative values. Thus, the image surface is excessively inclined towardthe plus value portion.

In this embodiment, there is provided the zoom lens for use suitably ina video camera having a solid-image sensing device on the image surfacethereof. To prevent change in the position of the exit pupil withrespect to the solid-image sensing device while maintaining appropriateback focus, change of the sign of the magnification β₄ of the fourthlens unit with respect to an infinite object is prevented duringzooming. If the magnification β₄ is changed undesirably simultaneouslywith the sign thereof, the angle of incident of light on the fixed fifthlens unit, which is the final lens unit, is undesirably changed. Theforegoing structure, however, prevents the change in the position of theexit pupil.

It is further preferable that, assuming that the paraxial horizontalmagnifications of the fourth lens unit at the wide angle end and thetelescopic end with respect to an infinite object are β_(4W) and β_(4T),respectively, setting be performed as follows:

    β.sub.4W ×β.sub.4T >0

To adequately maintaining the back focus of the lens while appropriatelyproviding the exit pupil of the optical system, it is preferable thatthe following be always satisfied:

    |β.sub.5 |<0.25

or

    0<β.sub.5 <0.25

If the value is out of the foregoing range, the back focus islengthened, the lens system is enlarged and the exit pupil is shortenedundesirably.

Furthermore, it is preferable that the following be satisfied assumingthat the focal length of the i-th lens unit is Fi, the focal lengths ofthe overall system at the wide angle end and the telescopic end areF_(W) and F_(t), respectively:

    0.25<F.sub.2 /F.sub.4 <0.7

The foregoing expression provides an optimum power configuration of themovable lens units. If the value is larger than the upper limit, theamount of movement of the second lens unit for obtaining a desiredvariable power ratio is enlarged excessively. As a result, the diameterof the front lens and the size of the overall system are enlargedexcessively. If the value is smaller than the lower limit, the amount ofmovement of the fourth lens unit, which is an image-surface correctingunit and is a focusing unit, is enlarged excessively. Thus, the size ofthe overall system is enlarged undesirably. The foregoing expression isrequired to provide an appropriate amount of movement of the movablelens units. Specifically, an appropriate amount of movement of thesecond lens unit is provided and the fourth lens unit is efficientlymoved in order to vary the power. In particular, the foregoingexpression is required to move the fourth lens unit from the wide angleend to the telescopic end to form a convex locus facing the imagesurface. More particularly, the expression is required to cause the sameto substantially reciprocate.

Assuming that the ratio (β_(2T) /β_(2W)) of the magnifications of thesecond lens unit from the wide angle end to the telescopic end is Z2 andthe ratio (F_(T) /F_(W)) of the focal lengths of the overall system isZ, it is preferable that the following be satisfied:

    0.4<Z2/Z<1.3

The foregoing expression shows the ratio of the power variation ratio ofthe second lens unit with respect to that of the overall system. If thevalue is larger than the upper limit, any of the units in the lenssystem decreases the magnification during zooming. Thus, the powervariation efficiency deteriorates and the size of the lens system isenlarged excessively. If the value is smaller than the lower limit, aplurality of lens units that contribute to the power variation exist. Inthis case, each lens unit cannot easily be controlled.

    0.2<|F.sub.2 |/√F.sub.W ×F.sub.1 <0.7

The foregoing expression relates to the power of the second lens unitwhich is the main unit for changing the magnification. By appropriatelyselecting a value in the foregoing range, the significant powervariation can be performed effectively. Specifically, if the value issmaller than the lower limit, the power of the second lens unit isstrengthened excessively, the Petzval sum is enlarged in a range ofnegative values, and thus excess image surface (excess correction) takesplace. What is worse, the sensitivity will be raised excessively, andout of focus and image shake easily take place. In this case, themechanism and structure are complicated excessively. If the value islarger than the upper limit, the power of the second lens unit isweakened, causing the amount of movement of the second lens unit forrealizing a desired variable power ratio to be enlarged. Thus, the sizecannot be reduced.

It is preferable that F2 be included in the following range in order toenlarge the magnification:

    0.2<|F2|/ √F.sub.W ×F.sub.1 <0.5

Furthermore, it is preferable that the following be satisfied:

    1.5<F.sub.3 /F.sub.W <4.5

    1.9<|F.sub.4 /F.sub.W |<5

    1.6<F.sub.5 /F.sub.W <5

The foregoing expressions relate to the power of the lens units disposedfrom the aperture to the image surface. Each range is set to prevent theposition of the exit pupil of the zoom lens from being shortened so asto cause diagonal beams from the object to be made incident on the pupilat a small angle as described above in order to reduce the diameter ofthe front lens.

If each value is larger than the upper limit, the diagonal beams cannotbe made incident at a small angle, and therefore the diameter of thefront lens is undesirably enlarged. If the value is smaller than thelower limit, the exit pupil is shortened excessively toward the positivevalues, thus resulting in that telecentric beams with respect to thesolid-image sensing device cannot be realized. What is worse, excessiveaberration change takes place when zooming or focusing is performed. Ifthe values are included in the foregoing ranges, a small size structurecan be realized with which rangefinding (focusing) can be performedwithout the excessive change. If the values are out of the foregoingranges, the diameter of the front lens and the size of the overallsystem are enlarged excessively. Furthermore, an appropriate emissionangle with respect to the solid-image sensing device cannot be provided.

It is preferable that the position of the exit pupil satisfies thefollowing:

    -1.5<F.sub.4 /F.sub.5 <-0.75

If the value is larger than the upper limit, the refracting power of thefourth lens unit is strengthened, whereby the focusing distance islengthened excessively. If the value is smaller than the lower limit,the amount of movement at the time of performing focusing is enlargedexcessively. Thus, desired size reduction cannot be performed. What isworse, the refracting power of the fifth lens unit, which is the fixedimaging lens, is strengthened excessively. Thus, the exit pupil isexcessively shortened.

To widen the field angle at the wide angle end, the distance e1 betweenthe principal points of the positive first lens unit and the negativesecond lens unit at the wide angle end must be shortened to furthershorten the focal length of the overall system to realize a wide-angle.A certain air gap must be provided between the first lens unit and thesecond lens unit to prevent contact between the two lens units.Accordingly, it is preferable that the principal point of the first lensunit adjacent the image be located adjacent to the second lens unit, orthe principal point of the second lens unit adjacent to the object belocated adjacent to the first lens unit. In particular, it is preferablethat the first lens unit satisfies the following conditional expressionto as well as reduce the diameter of the front lens:

    -0.25<H.sub.1 /F.sub.1 <-0.05

where H1' is the distance from the vertex of the surface of the firstlens unit nearest the image to the principal point of the first lensunit adjacent to the image (- indicates positions near the object,while + indicates positions near the image), and F₁ indicates the focallength of the first lens unit. The foregoing expression is required toshorten the distance from the principal point of the first lens unit tothat of the second lens unit. If the value is smaller than the lowerlimit, the principal point of the first lens unit is moved excessivelytoward the object. Thus, e1 cannot be shortened. In the foregoing case,a desired wide angle cannot be realized. If the value is larger than theupper limit, the distance from the first lens unit to the second lensunit is lengthened excessively. As a result, the diameter of the frontlens is enlarged excessively.

It is preferable that the following range be satisfied to reduce thediameter of the front lens:

    -0.10<H1'/F.sub.1 <-0.05

To further reduce the size of the overall system, it is preferable thatthe following be satisfied:

    1.5<Bf.sub.W /F.sub.W <4.0

where Bf_(W) is the back focus at the wide angle end (except an element,such as a glass block or a filter, represented by G) when the distancefrom the object is infinite. The foregoing expression is required toreduce effectively the size of the overall system. If the value issmaller than the lower limit, a block, such as a filter, cannot beinserted, and the exit pupil is shortened. As a result, the forming ofan image onto the image pickup device is undesirably out of thetelecentric system. If the value is larger than the upper limit, desiredsize reduction cannot be performed.

To reduce the diameter of the front lens, it is preferable that thefollowing be satisfied:

    4.5<F.sub.1 /F.sub.W <10

The foregoing expression relates to the object point with respect to thesecond lens unit, that is, to the magnification. To reduce the size ofthe overall system, it is preferable that the second lens unit performthe zooming while allowing the equal magnification to intervene. If theequal magnification is allowed to intervene, the locus of the zooming ofthe fourth lens unit is made to substantially reciprocate. Thus,significant power variation can be performed with the most effectivespace efficiency. Specifically, if the value is larger than the upperlimit, the object point with respect to the second lens unit is maderemote, the imaging magnification of the second lens unit is lowered,and desired size reduction cannot easily be performed. Furthermore, thedistance from the first lens unit to the second lens unit is lengthenedexcessively, and the size reduction cannot easily be performed. If thevalue is smaller than the lower limit, the magnification of the secondlens unit is enlarged and therefore large magnification cannot easily berealized.

To enlarge the magnification, it is preferable that the ratio beincluded in the following range:

    7.5<F.sub.1 /F.sub.W <10

As a specific lens structure, the first lens unit comprises a pluralityof lenses including at least one negative lens, the second lens unitcomprises a plurality of lenses including at least one positive lens,the third lens unit comprises at least one positive lens, the fourthlens unit comprises at least one negative lens, and the fifth lens unitcomprises a plurality of lenses including at least one negative lens.

In particular, it is preferable that the first lens unit comprises threelenses consisting of, when viewed from the object, a negative meniscuslens having a convex facing the object, a positive lens having a convexfacing the object, and a positive lens having a convex facing theobject. It is preferable that the second lens unit comprises, whenviewed from the object, a negative meniscus lens having a convex facingthe object, a double-convex lens and a positive lens.

If an aspherical surface is provided for each surface of the third lensunit, advanced aberration correction can be performed. In a case wherethe third lens unit is composed of one lens, the aspherical surface maybe provided for the surface facing the object or that facing the imagesurface. It is preferable that the aspherical surface be provided forthe surface facing the object to relatively satisfactorily correctspherical aberration.

To prevent change in the aberration occurring due to focusing, inparticular, to prevent change in the chromatic aberration, it ispreferable that the fourth lens unit be composed of a positive lens anda negative lens in such a manner that the positive lens and the negativelens are bonded to each other. It is preferable that the following besatisfied assuming that the Abbe's number of the positive lens and thatof the negative lens are ν4p and ν4n:

    ν4p<ν4n

If the foregoing relationship is not be held, the chromatic aberrationis changed excessively due to the distance.

If the fourth lens unit comprises the positive lens and the negativelens that are bonded to each other as described above, the fourth lensunit, which is moved when zooming is performed and when focusing isperformed, can be formed into one block. Thus, the structure can besimplified and the fourth lens unit can easily be supported.

Assuming that the Abbe's number of the positive lens and that of thenegative lens of the fourth lens unit are ν4p and ν4n, it is preferablethat the following be satisfied:

    ν4n-ν4p>15

To effectively change the chromatic aberration during zooming, it ispreferable that the following expression be satisfied:

    ν4n-ν4p>30

If the lenses composing the fourth lens unit are formed into one blockas described above, and particularly, if the positive lens and thenegative lens are bonded to each other or if the fourth lens unit iscomposed of one negative lens, it is preferable that the image surfaceof the block facing the object has a concave facing the object and thesurface facing the image has a concave facing the image.

The fifth lens unit is the positive lens unit, which is fixed duringzooming, and, therefore, it easily encounters spherical aberration. Tocorrect the spherical aberration, it is preferable that the fifth lensunit be composed of one or more positive lens and one or more negativelens. In a case where the fifth lens unit is composed of one positivelens and one negative lens, it is preferable that an aspherical surfacebe disposed on either of the surfaces. As a result, the sphericalaberration can satisfactorily be corrected. To correct chromaticaberration, a bonded lens unit may be disposed. If focusing is performedby the fifth lens unit, it is preferable that the fifth lens unit be anintegrated lens manufactured by bonding a negative lens and a positivelens because the weight of the lens can be reduced and its structure canbe simplified. Thus, the fifth lens unit can be supported easily.

As described above, when the angle is widen, it is an important factorto shorten, at the wide angle end, the distance e1 between the principalpoint of the first lens unit and that of the second lens unit. Toshorten e1, it is preferable that the shape of the first lens unit be asfollows:

The first lens unit comprises a negative meniscus lens L11 having aconvex facing the object, an air gap, a positive lens L12, and apositive lens L13 having a convex facing the object, the foregoingelements being disposed in the foregoing sequential order when viewedfrom the object. Furthermore, the air lens defined by the lenses L11 andL12 has negative refracting power.

To shorten e1 at the wide angle end, it is preferable that the secondlens unit has a structure such that its principal point facing theobject is disposed adjacent to the object.

Specifically, the second lens unit comprises a negative meniscus lensL21 having a convex facing the object, a double-concave negative lensL22, air gap, and a positive lens L23, the foregoing elements beingdisposed when viewed from the object. The foregoing air gap causes theprincipal point of the second lens unit facing the object to be movedtoward the first lens unit to shorten e1 at the wide angle end. Thus,the angle can effectively be widened. A negative lens L24 may bedisposed adjacent to the lens L23 on the side on which the lens L23faces the image. The lens L24 causes the principal point of the secondlens unit facing the object to be moved toward the first lens unit.Furthermore, change in the chromatic aberration occurring due to thezooming can be prevented.

A small size and wide angle zoom lens can be obtained by, in theforegoing structure, forming the structure to satisfy the following,assuming that the outer diameter of the lens nearest the object is φ1,and the focal length of the overall system at the wide angle end isF_(W) :

    0.10<F.sub.W /φ1<0.20

Generally, when the focal length F_(W) at the wide angle end isshortened, the diameter φ1 of the front lens is enlarged accordingly. Onthe contrary, if the focal length F_(W) at the wide angle end islengthened, the diameter φ1 of the front lens is reduced though thedegree depends upon the brightness of the telescopic end of the lens.The foregoing expression indicates an appropriate relationship betweenthe diameter of the front lens and the focal length at the wide angleend to realize a small size and wide angle zoom lens. If the foregoingratio is out of the foregoing range, a small size and wide angle zoomlens cannot be provided. Specifically, if the value is larger than theupper limit, a zoom lens like a telescopic zoom lens is realized. If thevalue is smaller than the lower limit, a zoom lens having a large sizeis realized.

The diameter φ1 of the front lens is substantially the opticallyeffective diameter of the first lens unit and is an actual outerdiameter of the lens. The diameter φ1 of the front lens is larger thanthe optically effective diameter of the first lens unit by 0% to 6%.

Although the fifth lens unit is caused to remain stationary duringzooming, it may be moved when focusing is performed.

Then, examples of the present invention will now be described.

In the examples, ri is the curvature radius of the i-th lens surfacecounted in a direction from an object, di is the thickness of the lensand the air gap counted in the direction from the object, and ni and νiare refracting power and Abbe's number of the i-th lens counted in thedirection from the object.

Under conditions that X-axis stands for the direction of the opticalaxis, Y-axis stands for the direction perpendicular to the optical axis,a direction in which light travels is positive, the intersection betweenthe vertex of the lens and the X-axis is made to be the original point,r is the paraxial curvature radius of the lens surface, and k, A₂, A₃,A₄ and A₅ are aspherical surface coefficients, the aspherical surface isexpressed by the following equation: ##EQU1##

Expression "D-0X" indicates "10^(-X) ". Symbol G represents a forceplate or the like.

EXAMPLE

    __________________________________________________________________________      f = 1˜19.57       Fno = 1.85˜3.59        2ω =             67.5°˜3.9°                                                __________________________________________________________________________        R1  = 13.101                                                                             D1  = 0.33                                                                              N1  = 1.846659                                                                        ν1  = 23.8                                    R2  = 6.721                                                                              D2  = 0.29                                                         R3  = 9.490                                                                              D3  = 0.95                                                                              N2  = 1.603112                                                                        ν2  = 60.7                                    R4  = -89.131                                                                            D4  = 0.05                                                         R5  = 5.414                                                                              D5  = 1.06                                                                              N3  = 1.603112                                                                        ν3  = 60.7                                    R6  = 88.678                                                                             D6  = VARIABLE                                                     R7  = 25.128                                                                             D7  = 0.16                                                                              N4  = 1882997                                                                         ν4  = 40.8                                    R8  = 1.503                                                                              D8  = 0.51                                                         R9  = -4.687                                                                             D9  = 0.16                                                                              N5  = 1.834807                                                                        ν5  = 42.7                                    R10 = 2.719                                                                              D10 = 0.27                                                         R11 = 3.171                                                                              D11 = 0.61                                                                              N6  = 1.846659                                                                        ν6  = 23.8                                    R12 = -7.254                                                                             D12 = 0.02                                                         R13 = -11.421                                                                            D13 = 0.16                                                                              N7  = 1.772499                                                                        ν7  = 49.6                                    R14 = 16.005                                                                             D14 = VARIABLE                                                     R15 = (DIAPHRAGM)                                                                        D15 = 0.46                                                         R16 = 7.963                                                                              D16 = 0.89                                                                              N8  = 1.603112                                                                        ν8  = 60.7                                    R17 = -4.826                                                                             D17 = 0.05                                                         R18 = 3.208                                                                              D18 = 0.81                                                                              N9  = 1.603112                                                                        ν9  = 60.7                                    R19 = -11.955                                                                            D19 = 0.07                                                         R20 = -6.226                                                                             D20 = 0.27                                                                              N10 = 1.834000                                                                        ν10 = 37.2                                    R21 = 2.723                                                                              D21 = 0.06                                                         R22 = 3.150                                                                              D22 = 0.69                                                                              N11 = 1.603112                                                                        ν11 = 60.7                                    R23 = -5.406                                                                             D23 = VARIABLE                                                     R24 = -5.280                                                                             D24 = 0.19                                                                              N12 = 1.772499                                                                        ν12 = 49.6                                    R25 = 5.174                                                                              D25 = VARIABLE                                                     R26 = 20.183                                                                             D26 = 0.82                                                                              N13 = 1.603112                                                                        ν13 = 60.7                                    R27 = -4.420                                                                             D27 = 0.05                                                         R28 = 3.072                                                                              D28 = 0.19                                                                              N14 = 1.846659                                                                        ν14 = 23.8                                    R29 = 2.090                                                                              D29 = 0.15                                                         R30 = 2.222                                                                              D30 = 0.82                                                                              N15 = 1.516330                                                                        ν15 = 64.2                                    R31 = -29.049                                                                            D31 = 0.55                                                          R32 = ∞                                                                            D32 = 1.18                                                                              N16 = 1.516330                                                                        ν16 = 64.2                                   R33 = ∞                                                             __________________________________________________________________________               FOCAL LENGTH                                                       VARIABLE DISTANCE                                                                        1.00        7.74                                                                             19.57                                               __________________________________________________________________________    D6         0.25        4.74                                                                             6.01                                                D14        6.23        1.74                                                                             0.47                                                D23        0.32        2.31                                                                             1.06                                                D25        2.75        0.76                                                                             2.01                                                __________________________________________________________________________

EXAMPLE

    __________________________________________________________________________      f = 1˜19.60       Fno = 1.85˜3.59        2ω =             67.6°˜3.9°                                                __________________________________________________________________________        R1  = 12.800                                                                             D1  = 0.33                                                                              N1  = 1.846659                                                                        ν1  = 23.8                                    R2  = 6.548                                                                              D2  = 0.24                                                         R3  = 8.744                                                                              D3  = 0.93                                                                              N2  = 1.603112                                                                        ν2  = 60.7                                    R4  = -91.307                                                                            D4  = 0.05                                                         R5  = 5.318                                                                              D5  = 1.05                                                                              N3  = 1.603112                                                                        ν3  = 60.7                                    R6  = 89.506                                                                             D6  = VARIABLE                                                     R7  = 68.327                                                                             D7  = 0.16                                                                              N4  = 1.882997                                                                        ν4  = 40.8                                    R8  = 1.718                                                                              D8  = 0.51                                                         R9  = -4.669                                                                             D9  = 0.16                                                                              N5  = 1.834807                                                                        ν5  = 42.7                                    R10 = 2.903                                                                              D10 = 0.27                                                         R11 = 3.294                                                                              D11 = 0.61                                                                              N6  = 1.846659                                                                        ν6  = 23.8                                    R12 = -6.563                                                                             D12 = 0.02                                                         R13 = -11.877                                                                            D13 = 0.16                                                                              N7  = 1.772499                                                                        ν7  = 49.6                                    R14 = 9.429                                                                              D14 = VARIABLE                                                     R15 = (DIAPHRAGM)                                                                        D15 = VARIABLE                                                     R16 = 8.067                                                                              D16 = 0.80                                                                              N8  = 1.603112                                                                        ν8  = 60.7                                    R17 = -5.091                                                                             D17 = 0.05                                                         R18 = 3.527                                                                              D18 = 0.82                                                                              N9  = 1.603112                                                                        ν9  = 60.7                                    R19 = -12.033                                                                            D19 = 0.09                                                         R20 = -6.103                                                                             D20 = 0.27                                                                              N10 = 1.834000                                                                        ν10 = 37.2                                    R21 = 2.842                                                                              D21 = 0.05                                                         R22 = 3.131                                                                              D22 = 0.72                                                                              N11 = 1.603112                                                                        ν11 = 60.7                                    R23 = -5.316                                                                             D23 = VARIABLE                                                     R24 = -5.373                                                                             D24 = 0.19                                                                              N12 = 1.772499                                                                        ν12 = 49.6                                    R25 = 5.527                                                                              D25 = VARIABLE                                                     R26 = 8.977                                                                              D26 = 0.82                                                                              N13 = 1.603112                                                                        ν13 = 60.7                                    R27 = -4.983                                                                             D27 = 0.05                                                         R28 = 3.004                                                                              D28 = 0.19                                                                              N14 = 1.846659                                                                        ν14 = 23.8                                    R29 = 2.084                                                                              D29 = 0.10                                                         R30 = 2.367                                                                              D30 = 0.90                                                                              N15 = 1.516330                                                                        ν15 = 64.2                                    R31 = 559.466                                                                            D31 = 0.55                                                          R32 = ∞                                                                            D32 = 1.19                                                                              N16 = 1.516330                                                                        ν16 = 64.2                                   R33 = ∞                                                             __________________________________________________________________________               FOCAL LENGTH                                                       VARIABLE DISTANCE                                                                        1.00        8.18                                                                             19.60                                               __________________________________________________________________________    D6         0.25        4.27                                                                             5.41                                                D14        5.53        1.50                                                                             0.37                                                D15        1.73        0.66                                                                             0.36                                                D23        0.32        2.85                                                                             2.09                                                D25        2.73        1.26                                                                             2.32                                                __________________________________________________________________________

EXAMPLE

    __________________________________________________________________________      f = 1˜20.05       Fno = 1.85˜3.59        2ω =             62.9°˜3.5°                                                __________________________________________________________________________        R1  = 15.447                                                                             D1  = 0.30                                                                              N1  = 1.846659                                                                        ν1  = 23.8                                    R2  = 7.375                                                                              D2  = 0.23                                                         R3  = 10.645                                                                             D3  = 0.84                                                                              N2  = 1.603112                                                                        ν2  = 60.7                                    R4  = -36.500                                                                            D4  = 0.05                                                         R5  = 5.434                                                                              D5  = 1.05                                                                              N3  = 1.603112                                                                        ν3  = 60.7                                    R6  = 53.752                                                                             D6  = VARIABLE                                                     R7  = 36.191                                                                             D7  = 0.15                                                                              N4  = 1.882997                                                                        ν4  = 40.8                                    R8  = 1.718                                                                              D8  = 0.50                                                         R9  = -5.552                                                                             D9  = 0.15                                                                              N5  = 1.834000                                                                        ν5  = 37.2                                    R10 = 3.669                                                                              D10 = 0.27                                                         R11 = 3.494                                                                              D11 = 0.62                                                                              N6  = 1.846659                                                                        ν6  = 23.8                                    R12 = -3.680                                                                             D12 = 0.04                                                         R13 = -3.363                                                                             D13 = 0.17                                                                              N7  = 1.785896                                                                        ν7  = 44.2                                    R14 = 15.751                                                                             D14 = VARIABLE                                                     R15 = (DIAPHRAGM)                                                                        D15 = VARIABLE                                                     R16 = 14.991                                                                             D16 = 0.68                                                                              N8  = 1.583126                                                                        ν8  = 59.4                                    R17 = -4.832                                                                             D17 = 0.05                                                         R18 = 7.603                                                                              D18 = 0.93                                                                              N9  = 1.719995                                                                        ν9  = 50.3                                    R19 = -2.730                                                                             D19 = 0.25                                                                              N10 = 1.846659                                                                        ν10 = 23.8                                    R20 = -10.068                                                                            D20 = VARIABLE                                                     R21 = -4.381                                                                             D21 = 0.17                                                                              N11 = 1.603112                                                                        ν11 = 60.7                                    R22 = 2.530                                                                              D22 = 0.50                                                                              N12 = 1.846659                                                                        ν12 = 23.8                                    R23 = 3.871                                                                              D23 = VARIABLE                                                     R24 = 12.568                                                                             D24 = 0.82                                                                              N13 = 1.583126                                                                        ν13 = 59.4                                    R25 = -4.243                                                                             D25 = 0.05                                                         R26 = 3.627                                                                              D26 = 0.17                                                                              N14 = 1.846659                                                                        ν14 = 23.8                                    R27 = 2.027                                                                              D27 = 0.12                                                         R28 = 2.151                                                                              D28 = 0.87                                                                              N15 = 1.516330                                                                        ν15 = 64.2                                    R29 = -32.087                                                                            D29 = 0.50                                                          R30 = ∞                                                                            D30 = 1.08                                                                              N16 = 1.516330                                                                        ν16 = 64.2                                   R31 = ∞                                                             __________________________________________________________________________               FOCAL LENGTH                                                       VARIABLE DISTANCE                                                                        1.00        8.45                                                                             20.05                                               __________________________________________________________________________    D6         0.22        4.59                                                                             5.82                                                D14        5.95        1.58                                                                             0.35                                                D15        1.76        0.79                                                                             0.52                                                D20        0.54        2.93                                                                             1.74                                                D23        2.54        1.12                                                                             2.59                                                __________________________________________________________________________

EXAMPLE

    __________________________________________________________________________      f = 1˜19.70       Fno = 1.65˜2.84        2ω =             62.1°˜3.5°                                                __________________________________________________________________________        R1  = 15.257                                                                             D1  = 0.30                                                                              N1  = 1.846659                                                                        ν1  = 23.8                                    R2  = 7.634                                                                              D2  = 0.36                                                         R3  = 10.870                                                                             D3  = 0.99                                                                              N2  = 1.603112                                                                        ν2  = 60.7                                    R4  = -84.464                                                                            D4  = 0.05                                                         R5  = 6.156                                                                              D5  = 1.33                                                                              N3  = 1.603112                                                                        ν3  = 60.7                                    R6  = 95.517                                                                             D6  = VARIABLE                                                     R7  = 22.162                                                                             D7  = 0.15                                                                              N4  = 1.882997                                                                        ν4  = 40.8                                    R8  = 1.597                                                                              D8  = 0.50                                                         R9  = -4.863                                                                             D9  = 0.15                                                                              N5  = 1.834000                                                                        ν5  = 37.2                                    R10 = 4.907                                                                              D10 = 0.27                                                         R11 = 3.779                                                                              D11 = 0.62                                                                              N6  = 1.846659                                                                        ν6  = 23.8                                    R12 = -4.675                                                                             D12 = 0.03                                                         R13 = -3.804                                                                             D13 = 0.17                                                                              N7  = 1.785896                                                                        ν7  = 44.2                                    R14 = 217.759                                                                            D14 = VARIABLE                                                     R15 = (DIAPHRAGM)                                                                        D15 = 0.42                                                         R16 = 53.443                                                                             D16 = 0.85                                                                              N8  = 1.583126                                                                        ν8  = 59.4                                    R17 = -4.258                                                                             D17 = 0.05                                                         R18 = 5.595                                                                              D18 = 1.05                                                                              N9  = 1.719995                                                                        ν9  = 50.3                                    R19 = -2.961                                                                             D19 = 0.25                                                                              N10 = 1.846659                                                                        ν10 = 23.8                                    R20 = -12.916                                                                            D20 = VARIABLE                                                     R21 = -4.385                                                                             D21 = 0.17                                                                              N11 = 1.603112                                                                        ν11 = 60.7                                    R22 = 1.992                                                                              D22 = 0.49                                                                              N12 = 1.846659                                                                        ν12 = 23.8                                    R23 = 3.225                                                                              D23 = VARIABLE                                                     R24 = 9.664                                                                              D24 = 0.75                                                                              N13 = 1.583126                                                                        ν13 = 59.4                                    R25 = -4.531                                                                             D25 = 0.05                                                         R26 = 3.530                                                                              D26 = 0.17                                                                              N14 = 1.846659                                                                        ν14 = 23.8                                    R27 = 1.790                                                                              D27 = 0.03                                                         R28 = 1.812                                                                              D28 = 1.06                                                                              N15 = 1.516330                                                                        ν15 = 64.2                                    R29 = -18.764                                                                            D29 = 0.49                                                          R30 = ∞                                                                            D30 = 1.07                                                                              N16 = 1.516330                                                                        ν16 = 64.2                                   R31 = ∞                                                             __________________________________________________________________________               FOCAL LENGTH                                                       VARIABLE DISTANCE                                                                        1.00        7.66                                                                             19.70                                               __________________________________________________________________________    D6         0.22        5.40                                                                             6.86                                                D14        7.28        2.11                                                                             0.65                                                D20        0.50        2.22                                                                             1.12                                                D23        2.68        0.96                                                                             2.06                                                __________________________________________________________________________

EXAMPLE

    __________________________________________________________________________      f = 1˜19.48       Fno = 1.65˜3.65        2ω =             62.0°˜3.5°                                                __________________________________________________________________________        R1  = 15.492                                                                             D1  = 0.29                                                                              N1  = 1.846659                                                                        ν1  = 23.8                                    R2  = 7.600                                                                              D2  = 0.22                                                         R3  = 11.946                                                                             D3  = 0.74                                                                              N2  = 1.603112                                                                        ν2  = 60.7                                    R4  = -34.062                                                                            D4  = 0.05                                                         R5  = 5.459                                                                              D5  = 0.94                                                                              N3  = 1.603112                                                                        ν3  = 60.7                                    R6  = 45.879                                                                             D6  = VARIABLE                                                     R7  = 18.340                                                                             D7  = 0.15                                                                              N4  = 1.882997                                                                        ν4  = 40.8                                    R8  = 1.431                                                                              D8  = 0.50                                                         R9  = -2.391                                                                             D9  = 0.15                                                                              N5  = 1.804000                                                                        ν5  = 46.6                                    R10 = 8.122                                                                              D10 = 0.21                                                         R11 = 5.326                                                                              D11 = 0.39                                                                              N6  = 1.846659                                                                        ν6  = 23.8                                    R12 = -6.425                                                                             D12 = VARIABLE                                                     R13 = (DIAPHRAGM)                                                                        D13 = 0.32                                                         R14 = 10.085                                                                             D14 = 0.74                                                                              N7  = 1.638539                                                                        ν7  = 55.4                                    R15 = -4.741                                                                             D15 = 0.05                                                         R16 = 6.192                                                                              D16 = 1.14                                                                              N8  = 1.719995                                                                        ν8  = 50.3                                    R17 = -2.338                                                                             D17 = 0.17                                                                              N9  = 1.846659                                                                        ν9  = 23.8                                    R18 = -11.140                                                                            D18 = VARIABLE                                                     R19 = -3.115                                                                             D19 = 0.52                                                                              N10 = 1.846659                                                                        ν10 = 23.8                                    R20 = -1.961                                                                             D20 = 0.17                                                                              N11 = 1.603112                                                                        ν11 = 60.7                                    R21 = 3.973                                                                              D21 = VARIABLE                                                     R22 = 8.417                                                                              D22 = 0.76                                                                              N12 = 1.583126                                                                        ν12 = 59.4                                    R23 = -3.992                                                                             D23 = 0.05                                                         R24 = 2.907                                                                              D24 = 0.15                                                                              N13 = 1.846659                                                                        ν13 = 23.8                                    R25 = 1.605                                                                              D25 = 0.10                                                         R26 = 1.678                                                                              D26 = 0.90                                                                              N14 = 1.516330                                                                        ν14 = 64.2                                    R27 = -62.998                                                                            D27 = 0.49                                                          R28 = ∞                                                                            D28 = 1.07                                                                              N15 = 1.516330                                                                        ν15 = 64.2                                   R29 = ∞                                                             __________________________________________________________________________               FOCAL LENGTH                                                       VARIABLE DISTANCE                                                                        1.00        8.08                                                                             19.48                                               __________________________________________________________________________    D6         0.22        5.20                                                                             6.60                                                D12        6.66        1.68                                                                             0.28                                                D18        0.55        1.94                                                                             0.01                                                D21        2.12        0.73                                                                             2.67                                                __________________________________________________________________________

EXAMPLE

    __________________________________________________________________________      f = 1˜19.89       Fno = 1.85˜3.59        2ω =             62.9°˜3.5°                                                __________________________________________________________________________        R1  = 9.431                                                                              D1  = 0.30                                                                              N1  = 1.846659                                                                        ν1  = 23.8                                    R2  = 5.526                                                                              D2  = 0.47                                                         R3  = 11.054                                                                             D3  = 0.67                                                                              N2  = 1.603112                                                                        ν2  = 60.7                                    R4  = 73.823                                                                             D4  = 0.05                                                         R5  = 5.258                                                                              D5  = 1.25                                                                              N3  = 1.603112                                                                        ν3  = 60.7                                    R6  = -53.851                                                                            D6  = VARIABLE                                                     R7  = 313.377                                                                            D7  = 0.15                                                                              N4  = 1.882997                                                                        ν4  = 40.8                                    R8  = 1.345                                                                              D8  = 0.51                                                         R9  = -3.226                                                                             D9  = 0.15                                                                              N5  = 1.834807                                                                        ν5  = 42.7                                    R10 = 8.593                                                                              D10 = 0.15                                                         R11 = 3.561                                                                              D11 = 0.62                                                                              N6  = 1.846659                                                                        ν6  = 23.8                                    R12 = -5.519                                                                             D12 = 0.17                                                         R13 = -21.441                                                                            D13 = 0.17                                                                              N7  = 1.834801                                                                        ν7  = 42.7                                    R14 = 11.427                                                                             D14 = VARIABLE                                                     R15 = (DIAPHRAGM)                                                                        D15 = 0.35                                                         R16 = 12.921                                                                             D16 = 0.89                                                                              N8  = 1.516330                                                                        ν8  = 64.2                                    R17 = -3.641                                                                             D17 = 0.04                                                         R18 = 4.127                                                                              D18 = 0.85                                                                              N9  = 1.603112                                                                        ν9  = 60.7                                    R19 = -4.391                                                                             D19 = 0.03                                                         R20 = -3.946                                                                             D20 = 0.20                                                                              N10 = 1.846659                                                                        ν10 = 23.8                                    R21 = -16.696                                                                            D21 = VARIABLE                                                     R22 = -6.000                                                                             D22 = 0.50                                                                              N11 = 1.805177                                                                        ν11 = 25.4                                    R23 = -2.018                                                                             D23 = 0.17                                                                              N12 = 1.834807                                                                        ν12 = 42.7                                    R24 = 8.444                                                                              D24 = VARIABLE                                                     R25 = 8.240                                                                              D25 = 0.75                                                                              N13 = 1.603112                                                                        ν13 = 60.7                                    R26 = -4.299                                                                             D26 = 0.05                                                         R27 = 3.551                                                                              D27 = 0.17                                                                              N14 = 1.846659                                                                        ν14 = 23.8                                    R28 = 1.825                                                                              D28 = 0.08                                                         R29 = 2.072                                                                              D29 = 0.75                                                                              N15 = 1.603112                                                                        ν15 = 60.7                                    R30 = 19.436                                                                             D30 = 0.50                                                          R31 = ∞                                                                            D31 = 1.08                                                                              N16 = 1.516330                                                                        ν16 = 64.2                                   R32 = ∞                                                             __________________________________________________________________________               FOCAL LENGTH                                                       VARIABLE DISTANCE                                                                        1.00        7.75                                                                             19.89                                               __________________________________________________________________________    D6         0.22        5.05                                                                             6.41                                                D14        6.54        1.72                                                                             0.36                                                D21        0.50        2.41                                                                             0.84                                                D24        2.56        0.64                                                                             2.22                                                __________________________________________________________________________

EXAMPLE

    __________________________________________________________________________      f = 1˜19.73       Fno = 1.85˜3.59        2ω =             62.6°˜3.5°                                                __________________________________________________________________________        R1  = 13.356                                                                             D1  = 0.30                                                                              N1  = 1.846659                                                                        ν1  = 23.8                                    R2  = 6.662                                                                              D2  = 1.03                                                                              N2  = 1.603112                                                                        ν2  = 60.7                                    R3  = -54.071                                                                            D3  = 0.05                                                         R4  = 5.947                                                                              D4  = 0.75                                                                              N3  = 1.696797                                                                        ν3  = 55.5                                    R5  = 16.445                                                                             D5  = VARIABLE                                                     R6  = 50.069                                                                             D6  = 0.15                                                                              N4  = 1.882997                                                                        ν4  = 40.8                                    R7  = 1.413                                                                              D7  = 0.55                                                         R8  = -3.031                                                                             D8  = 0.15                                                                              N5  = 1.804000                                                                        ν5  = 46.6                                    R9  = 5.669                                                                              D9  = 0.23                                                         R10 = 4.087                                                                              D10 = 0.50                                                                              N6  = 1.846659                                                                        ν6  = 23.8                                    R11 = -5.071                                                                             D11 = 0.05                                                         R12 = -6.109                                                                             D12 = 0.20                                                                              N7  = 1.772499                                                                        ν7  = 49.6                                    R13 = 439.693                                                                            D13 = VARIABLE                                                     R14 = (DIAPHRAGM)                                                                        D14 = 0.48                                                         R15 = 33.345                                                                             D15 = 0.72                                                                              N8  = 1.583126                                                                        ν8  = 59.4                                    R16 = -3.950                                                                             D16 = 0.04                                                         R17 = 4.765                                                                              D17 = 0.99                                                                              N9  = 1.696791                                                                        ν9  = 55.5                                    R18 = -2.817                                                                             D18 = 0.20                                                                              N10 = 1.846659                                                                        ν10 = 23.8                                    R19 = -12.811                                                                            D19 = VARIABLE                                                     R20 = -4.773                                                                             D20 = 0.17                                                                              N11 = 1.712995                                                                        ν11 = 53.8                                    R21 = 2.392                                                                              D21 = 0.45                                                                              N12 = 1.846659                                                                        ν12 = 23.8                                    R22 = 4.652                                                                              D22 = VARIABLE                                                     R23 = 16.777                                                                             D23 = 0.74                                                                              N13 = 1.583126                                                                        ν13 = 59.4                                    R24 = -4.722                                                                             D24 = 0.05                                                         R25 = 10.243                                                                             D25 = 0.17                                                                              N14 = 1.805181                                                                        ν14 = 25.4                                    R26 = 2.560                                                                              D26 = 0.09                                                         R27 = 3.559                                                                              D27 = 0.74                                                                              N15 = 1.696797                                                                        ν15 = 55.5                                    R28 = -4.763                                                                             D28 = 0.50                                                          R29 = ∞                                                                            D29 = 1.08                                                                              N16 = 1.516330                                                                        ν16 = 64.2                                   R30 = ∞                                                             __________________________________________________________________________               FOCAL LENGTH                                                       VARIABLE DISTANCE                                                                        1.00        7.27                                                                             19.73                                               __________________________________________________________________________    D5         0.22        4.81                                                                             6.10                                                D13        6.54        1.96                                                                             0.66                                                D19        0.49        1.99                                                                             0.96                                                D22        2.40        0.90                                                                             1.92                                                __________________________________________________________________________

EXAMPLE

    __________________________________________________________________________      f = 1˜19.89       Fno = 1.65˜3.69        2ω =             62.1°˜3.5°                                                __________________________________________________________________________         R1  = 14.031                                                                            D1  = 0.29                                                                              N1  = 1.846659                                                                        ν1  = 23.8                                     R2  = 7.255                                                                             D2  = 0.17                                                          R3  = 10.275                                                                            D3  = 0.71                                                                              N2  = 1.603112                                                                        ν2  = 60.7                                     R4  = -73.155                                                                           D4  = 0.05                                                          R5  = 5.592                                                                             D5  = 0.91                                                                              N3  = 1.603112                                                                        ν3  = 60.7                                     R6  = 56.632                                                                            D6  = VARIABLE                                                      R7  = 12.094                                                                            D7  = 0.15                                                                              N4  = 1.882997                                                                        ν4  = 40.8                                     R8  = 1.481                                                                             D8  = 0.53                                                          R9  = -2.118                                                                            D9  = 0.15                                                                              N5  = 1.804000                                                                        ν5  = 46.6                                     R10 = 6.456                                                                             D10 = 0.21                                                          R11 = 5.707                                                                             D11 = 0.44                                                                              N6  = 1.846659                                                                        ν6  = 23.8                                     R12 = -5.707                                                                            D12 = VARIABLE                                                      R13 = (DIAPHRAGM)                                                                       D13 = 0.32                                                          R14 = 25.382                                                                            D14 = 0.64                                                                              N7  = 1.719995                                                                        ν7  = 50.3                                     R15 = -4.947                                                                            D15 = 0.05                                                          R16 = 6.790                                                                             D16 = 1.18                                                                              N8  = 1.719995                                                                        ν8  = 50.3                                     R17 = -2.463                                                                            D17 = 0.17                                                                              N9  = 1.846659                                                                        ν9  = 23.8                                     R18 = -9.436                                                                            D18 = VARIABLE                                                      R19 = -3.313                                                                            D19 = 0.49                                                                              N10 = 1.846659                                                                        ν10 = 23.8                                     R20 = -20.86                                                                            D20 = 0.17                                                                              N11 = 1.603112                                                                        ν11 = 60.7                                     R21 = 5.636                                                                             D21 = VARIABLE                                                      R22 = 9.117                                                                             D22 = 0.74                                                                              N12 = 1.583126                                                                        ν12 = 59.4                                    *R23 = -3.865                                                                            D23 = 0.05                                                          R24 = 2.938                                                                             D24 = 0.15                                                                              N13 = 1.846659                                                                        ν13 = 23.8                                     R25 = 1.509                                                                             D25 = 0.02                                                          R26 = 1.542                                                                             D26 = 0.98                                                                              N14 = 1.518229                                                                        ν14 = 59.0                                     R27 = 24.557                                                                            D27 = 0.49                                                           R28 = ∞                                                                           D28 = 0.91                                                                              N15 = 1.516330                                                                        ν15 = 64.2                                    R29 = ∞                                                            __________________________________________________________________________               FOCAL LENGTH                                                       VARIABLE DISTANCE                                                                        1.00        8.01                                                                             19.89                                               __________________________________________________________________________    D6         0.22        5.20                                                                             6.61                                                D12        6.68        1.70                                                                             0.29                                                D18        0.52        2.39                                                                             0.52                                                D21        2.87        1.00                                                                             2.87                                                __________________________________________________________________________    ASPHERICAL COEFFICIENT                                                        __________________________________________________________________________    R23   K = -5.18006e - 01                                                                       B = 2.88359e - 04                                                                        C = 1.63206e - 04                                       D =   8.64643e - 05                                                                      E = 4.65602e - 05                                            __________________________________________________________________________

EXAMPLE

    __________________________________________________________________________      f = 1˜19.87       Fno = 1.65˜3.69        2ω =             62.1°˜3.5°                                                __________________________________________________________________________        R1  = 17.601                                                                             D1  = 0.29                                                                              N1  = 1.846659                                                                        ν1  = 23.8                                    R2  = 8.134                                                                              D2  = 0.10                                                         R3  = 9.961                                                                              D3  = 0.81                                                                              N2  = 1.603112                                                                        ν2  = 60.7                                    R4  = -30.879                                                                            D4  = 0.05                                                         R5  = 5.593                                                                              D5  = 0.81                                                                              N3  = 1.603112                                                                        ν3  = 60.7                                    R6  = 24.342                                                                             D6  = VARIABLE                                                     R7  = 9.547                                                                              D7  = 0.15                                                                              N4  = 1.882997                                                                        ν4  = 40.8                                    R8  = 1.497                                                                              D8  = 0.53                                                         R9  = -2.051                                                                             D9  = 0.15                                                                              N5  = 1.804000                                                                        ν5  = 46.6                                    R10 = 5.740                                                                              D10 = 0.21                                                         R11 = 5.624                                                                              D11 = 0.44                                                                              N6  = 1.846659                                                                        ν6  = 23.8                                    R12 = -5.624                                                                             D12 = VARIABLE                                                     R13 = (DIAPHRAGM)                                                                        D13 = 0.32                                                         R14 = 400.612                                                                            D14 = 0.64                                                                              N7  = 1.719995                                                                        ν7  = 50.3                                    R15 = -4.836                                                                             D15 = 0.05                                                         R16 = 6.637                                                                              D16 = 1.18                                                                              N8  = 1.719995                                                                        ν8  = 50.3                                    R17 = 2.477                                                                              D17 = 0.17                                                                              N9  = 1.846659                                                                        ν9  = 23.8                                    R18 = -8.179                                                                             D18 = VARIABLE                                                     R19 = -3.442                                                                             D19 = 0.49                                                                              N10 = 1.846659                                                                        ν10 = 23.8                                    R20 = -2.129                                                                             D20 = 0.17                                                                              N11 = 1.603112                                                                        ν11 = 60.7                                    R21 = 6.542                                                                              D21 = VARIABLE                                                     R22 = 8.308                                                                              D22 = 0.69                                                                              N12 = 1.516330                                                                        ν12 = 64.2                                    R23 = -4.310                                                                             D23 = 0.05                                                         R24 = 2.950                                                                              D24 = 0.15                                                                              N13 = 1.846659                                                                        ν13 = 23.8                                    R25 = 1.460                                                                              D25 = 1.08                                                                              N14 = 1.517417                                                                        ν14 = 52.4                                    R26 = -35.689                                                                            D26 = 0.49                                                          R27 = ∞                                                                            D27 = 0.91                                                                              N15 = 1.516330                                                                        ν15 = 64.2                                   R28 = ∞                                                             __________________________________________________________________________               FOCAL LENGTH                                                       VARIABLE DISTANCE                                                                        1.00        7.94                                                                             19.87                                               __________________________________________________________________________    D6         0.22        5.20                                                                             6.61                                                D12        6.68        1.70                                                                             0.29                                                D18        0.52        2.49                                                                             0.50                                                D21        2.99        1.02                                                                             3.01                                                __________________________________________________________________________

EXAMPLE

    __________________________________________________________________________      f = 1˜19.87       Fno = 1.65˜3.69        2ω =             62.1°˜3.5°                                                __________________________________________________________________________        R1  = 16.732                                                                             D1  = 0.29                                                                              N1  = 1.846659                                                                        ν1  = 23.8                                    R2  = 7.953                                                                              D2  = 0.05                                                         R3  = 8.799                                                                              D3  = 0.86                                                                              N2  = 1.603112                                                                        ν2  = 60.7                                    R4  = -32.308                                                                            D4  = 0.04                                                         R5  = 5.653                                                                              D5  = 0.76                                                                              N3  = 1.603112                                                                        ν3  = 60.7                                    R6  = 19.248                                                                             D6  = VARIABLE                                                     R7  = 9.166                                                                              D7  = 0.15                                                                              N4  = 1.882997                                                                        ν4  = 40.8                                    R8  = 1.503                                                                              D8  = 0.53                                                         R9  = -2.044                                                                             D9  = 0.15                                                                              N5  = 1.804000                                                                        ν5  = 46.6                                    R10 = 5.523                                                                              D10 = 0.21                                                         R11 = 5.616                                                                              D11 = 0.44                                                                              N6  = 1.846659                                                                        ν6  = 23.8                                    R12 = -5.616                                                                             D12 = VARIABLE                                                     R13 = (DIAPHRAGM)                                                                        D13 = 0.32                                                         R14 = 73.706                                                                             D14 = 0.61                                                                              N7  = 1.719995                                                                        ν7  = 50.3                                    R15 = -4.809                                                                             D15 = 0.05                                                         R16 = 5.495                                                                              D16 = 1.23                                                                              N8  = 1.719995                                                                        ν8  = 50.3                                    R17 = -2.423                                                                             D17 = 0.17                                                                              N9  = 1.846659                                                                        ν9  = 23.8                                    R18 = -8.359                                                                             D18 = VARIABLE                                                     R19 = -3.083                                                                             D19 = 0.49                                                                              N10 = 1.846659                                                                        ν10 = 23.8                                    R20 = -1.899                                                                             D20 = 0.17                                                                              N11 = 1.603112                                                                        ν11 = 60.7                                    R21 = 3.989                                                                              D21 = VARIABLE                                                     R22 = 9.068                                                                              D22 = 0.69                                                                              N12 = 1.516330                                                                        ν12 = 64.2                                    R23 = -3.667                                                                             D23 = 0.05                                                         R24 = 3.030                                                                              D24 = 0.15                                                                              N13 = 1.846659                                                                        ν13 = 23.8                                    R25 = 1.459                                                                              D25 = 1.13                                                                              N14 = 1.517417                                                                        ν14 = 52.4                                    R26 = -12.233                                                                            D26 = 0.49                                                          R27 = ∞                                                                            D27 = 0.91                                                                              N15 = 1.516330                                                                        ν15 = 64.2                                   R28 = ∞                                                             __________________________________________________________________________               FOCAL LENGTH                                                       VARIABLE DISTANCE                                                                        1.00        7.91                                                                             19.87                                               __________________________________________________________________________    D6         0.22        5.20                                                                             6.61                                                D12        6.68        1.70                                                                             0.29                                                D18        0.58        2.08                                                                             0.61                                                D21        2.45        0.95                                                                             2.42                                                __________________________________________________________________________

EXAMPLE

    __________________________________________________________________________      f = 1˜19.89       Fno = 1.65˜3.69        2ω =             62.2°˜3.5°                                                __________________________________________________________________________         R1  = 15.881                                                                            D1  = 0.30                                                                              N1  = 1.846659                                                                        ν1  = 23.8                                     R2  = 7.848                                                                             D2  = 0.03                                                          R3  = 8.343                                                                             D3  = 0.91                                                                              N2  = 1.603112                                                                        ν2  = 60.7                                     R4  = -32.225                                                                           D4  = 0.04                                                          R5  = 5.734                                                                             D5  = 0.71                                                                              N3  = 1.603112                                                                        ν3  = 60.7                                     R6  = 17.103                                                                            D6  = VARIABLE                                                      R7  = 8.558                                                                             D7  = 0.15                                                                              N4  = 1.882997                                                                        ν4  = 40.8                                     R8  = 1.523                                                                             D8  = 0.53                                                          R9  = -2.005                                                                            D9  = 0.15                                                                              N5  = 1.804000                                                                        ν5  = 46.6                                     R10 = 5.668                                                                             D10 = 0.21                                                          R11 = 5.729                                                                             D11 = 0.44                                                                              N6  = 1.846659                                                                        ν6  = 23.8                                     R12 = -5.729                                                                            D12 = VARIABLE                                                      R13 = (DIAPHRAGM)                                                                       D13 = 0.32                                                          R14 = 149.670                                                                           D14 = 0.62                                                                              N7  = 1.719995                                                                        ν7  = 50.3                                     R15 = -4.474                                                                            D15 = 0.05                                                          R16 = 5.631                                                                             D16 = 1.21                                                                              N8  = 1.696797                                                                        ν8  = 55.5                                     R17 = -2.540                                                                            D17 = 0.17                                                                              N9  = 1.846659                                                                        ν9  = 23.8                                     R18 = -8.708                                                                            D18 = VARIABLE                                                      R19 = -3.121                                                                            D19 = 0.49                                                                              N10 = 1.846659                                                                        ν10 = 23.8                                     R20 = -1.891                                                                            D20 = 0.15                                                                              N11 = 1.603112                                                                        ν11 = 60.7                                     R21 = 5.793                                                                             D21 = VARIABLE                                                      R22 = 3.776                                                                             D22 = 0.15                                                                              N12 = 1.846659                                                                        ν12 = 23.8                                     R23 = 1.791                                                                             D23 = 1.33                                                                              N13 = 1.583126                                                                        ν13 = 59.4                                    *R24 = -2.525                                                                            D24 = 0.49                                                           R25 = ∞                                                                           D25 = 0.91                                                                              N14 = 1.516330                                                                        ν14 = 64.2                                    R26 = ∞                                                            __________________________________________________________________________               FOCAL LENGTH                                                       VARIABLE DISTANCE                                                                        1.00        7.73                                                                             19.89                                               __________________________________________________________________________    D6         0.22        5.22                                                                             6.63                                                D12        6.70        1.70                                                                             0.30                                                D18        0.55        2.27                                                                             0.58                                                D21        3.05        1.34                                                                             3.03                                                __________________________________________________________________________    ASPHERICAL COEFFICIENT                                                        __________________________________________________________________________    R24   K = -4.25591e - 01                                                                       B = 1.61244e - 02                                                                        C = -1.58627e - 03                                      D = -1.44598e - 03                                                                       E = 4.93025e - 04                                            __________________________________________________________________________

EXAMPLE

    __________________________________________________________________________      f = 1˜19.87       Fno = 1.65˜3.69        2ω =             62.1°˜3.5°                                                __________________________________________________________________________        R1  = 14.256                                                                             D1  = 0.29                                                                              N1  = 1.846659                                                                        ν1  = 23.8                                    R2  = 7.250                                                                              D2  = 0.17                                                         R3  = 10.339                                                                             D3  = 0.71                                                                              N2  = 1.603112                                                                        ν2  = 60.7                                    R4  = -66.464                                                                            D4  = 0.05                                                         R5  = 5.618                                                                              D5  = 0.91                                                                              N3  = 1.603112                                                                        ν3  = 60.7                                    R6  = 65.684                                                                             D6  = VARIABLE                                                     R7  = 11.097                                                                             D7  = 0.15                                                                              N4  = 1.882997                                                                        ν4  = 40.8                                    R8  = 1.477                                                                              D8  = 0.53                                                         R9  = -2.082                                                                             D9  = 0.15                                                                              N5  = 1.804000                                                                        ν5  = 46.6                                    R10 = 5.858                                                                              D10 = 0.21                                                         R11 = 5.487                                                                              D11 = 0.44                                                                              N6  = 1.846659                                                                        ν6  = 23.8                                    R12 = -5.487                                                                             D12 = VARIABLE                                                     R13 = (DIAPHRAGM)                                                                        D13 = 0.32                                                         R14 = 20.695                                                                             D14 = 0.64                                                                              N7  = 1.719995                                                                        ν7  = 50.3                                    R15 = -5.037                                                                             D15 = 0.05                                                         R16 = 6.705                                                                              D16 = 1.15                                                                              N8  = 1.719995                                                                        ν8  = 50.3                                    R17 = -2.398                                                                             D17 = 0.17                                                                              N9  = 1.846659                                                                        ν9  = 23.8                                    R18 = -9.388                                                                             D18 = VARIABLE                                                     R19 = -3.291                                                                             D19 = 0.49                                                                              N10 = 1.846659                                                                        ν10 = 23.8                                    R20 = -2.033                                                                             D20 = 0.17                                                                              N11 = 1.603112                                                                        ν11 = 60.7                                    R21 = 5.637                                                                              D21 = VARIABLE                                                     R22 = 8.844                                                                              D22 = 0.74                                                                              N12 = 1.583126                                                                        ν12 = 59.4                                    R23 = -4.236                                                                             D23 = 0.05                                                         R24 = 2.977                                                                              D24 = 0.15                                                                              N13 = 1.846659                                                                        ν13 = 23.8                                    R25 = 1.502                                                                              D25 = 0.02                                                         R26 = 1.540                                                                              D26 = 0.98                                                                              N14 = 1.518229                                                                        ν14 = 59.0                                    R27 = 63.256                                                                             D27 = 0.49                                                          R28 = ∞                                                                            D28 = 1.07                                                                              N15 = 1.516330                                                                        ν15 = 64.2                                   R29 = ∞                                                             __________________________________________________________________________               FOCAL LENGTH                                                       VARIABLE DISTANCE                                                                        1.00        7.96                                                                             19.87                                               __________________________________________________________________________    D6         0.22        5.20                                                                             6.61                                                D12        6.68        1.70                                                                             0.29                                                D18        0.47        2.24                                                                             0.39                                                D21        2.51        0.73                                                                             2.59                                                __________________________________________________________________________

EXAMPLE

    __________________________________________________________________________      f = 1˜19.87       Fno = 1.65˜3.69        2ω =             62.1°˜3.5°                                                __________________________________________________________________________         R1  = 14.160                                                                            D1  = 0.29                                                                              N1  = 1.846659                                                                        ν1  = 23.8                                     R2  = 7.259                                                                             D2  = 0.17                                                          R3  = 10.317                                                                            D3  = 0.71                                                                              N2  = 1.603112                                                                        ν2  = 60.7                                     R4  = -67.467                                                                           D4  = 0.05                                                          R5  = 5.595                                                                             D5  = 0.91                                                                              N3  = 1.603112                                                                        ν3  = 60.7                                     R6  = 59.378                                                                            D6  = VARIABLE                                                      R7  = 12.010                                                                            D7  = 0.15                                                                              N4  = 1.882997                                                                        ν4  = 40.8                                     R8  = 1.485                                                                             D8  = 0.53                                                          R9  = -2.113                                                                            D9  = 0.15                                                                              N5  = 1.804000                                                                        ν5  = 46.6                                     R10 = 6.321                                                                             D10 = 0.21                                                          R11 = 5.644                                                                             D11 = 0.44                                                                              N6  = 1.846659                                                                        ν6  = 23.8                                     R12 = -5.644                                                                            D12 = VARIABLE                                                      R13 = (DIAPHRAGM)                                                                       D13 = 0.32                                                          R14 = 24.830                                                                            D14 = 0.64                                                                              N7  = 1.719995                                                                        ν7  = 50.3                                     R15 = -5.022                                                                            D15 = 0.05                                                          R16 = 6.669                                                                             D16 = 1.18                                                                              N8  = 1.719995                                                                        ν8  = 50.3                                     R17 = -2.456                                                                            D17 = 0.17                                                                              N9  = 1.846659                                                                        ν9  = 23.8                                     R18 = -9.449                                                                            D18 = VARIABLE                                                      R19 = -3.270                                                                            D19 = 0.49                                                                              N10 = 1.846659                                                                        ν10 = 23.8                                     R20 = -2.059                                                                            D20 = 0.17                                                                              N11 = 1.603112                                                                        ν11 = 60.7                                     R21 = 5.687                                                                             D21 = VARIABLE                                                      R22 = 9.120                                                                             D22 = 0.74                                                                              N12 = 1.583126                                                                        ν12 = 59.4                                    *R23 = -3.899                                                                            D23 = 0.05                                                          R24 = 2.953                                                                             D24 = 0.15                                                                              N13 = 1.846659                                                                        ν13 = 23.8                                     R25 = 1.507                                                                             D25 = 0.02                                                          R26 = 1.544                                                                             D26 = 0.98                                                                              N14 = 1.518229                                                                        ν14 = 59.0                                     R27 = 26.939                                                                            D27 = 0.49                                                           R28 = ∞                                                                           D28 = 1.07                                                                              N15 = 1.516330                                                                        ν15 = 64.2                                    R29 = ∞                                                            __________________________________________________________________________               FOCAL LENGTH                                                       VARIABLE DISTANCE                                                                        1.00        8.03                                                                             19.87                                               __________________________________________________________________________    D6         0.22        5.20                                                                             6.61                                                D12        6.68        1.70                                                                             0.29                                                D18        0.53        2.34                                                                             0.38                                                D21        2.54        0.73                                                                             2.69                                                __________________________________________________________________________    ASPHERICAL COEFFICIENT                                                        __________________________________________________________________________    R23   K = -4.71958e - 01                                                                       B = -6.66698e - 05                                                                       C = 1.53449e - 04                                       D =   7.24360e - 05                                                                      E = -5.52728e - 05                                           __________________________________________________________________________

    __________________________________________________________________________           EXAMPLE 1                                                                            EXAMPLE 2                                                                            EXAMPLE 3                                                                            EXAMPLE 4                                                                            EXAMPLE 5                                                                            EXAMPLE 6                                                                            EXAMPLE                      __________________________________________________________________________                                                     7                            D.sub.4 w/Fw                                                                         2.75   2.73   2.54   2.67   2.12   2.56   2.40                         D.sub.2 w/D.sub.4 w                                                                  2.43   2.86   3.03   2.87   2.29   2.69   2.93                         D.sub.4 w/F.sub.4                                                                    -0.82  -0.78  -0.67  -0.75  -0.64  -0.65  -0.67                        F.sub.1 /F.sub.2                                                                     -6.43  -5.99  -5.97  -6.41  -5.76  -6.15  -6.54                        β.sub.5                                                                         0.06   0.03   ⃡0                                                                       0.05   0.11   0.15   0.15                         F.sub.2 /F.sub.4                                                                     0.40   0.40   0.38   0.43   0.47   0.36   0.38                         Z.sub.2 /Z                                                                           6.70   -0.49  0.58   0.71   1.68   0.83   0.76                          ##STR1##                                                                            -0.30  -0.31  -0.32  -0.34  -0.35  -0.32  -0.30                        F.sub.3 /Fw                                                                          3.35   3.50   3.58   3.44   3.12   3.33   3.16                         F.sub.4 /Fw                                                                          -3.36  -3.5   -3.78  -3.56  -3.29  -3.95  -3.55                        F.sub.5 /Fw                                                                          3.52   3.57   3.95   3.80   3.41   3.94   3.94                         F.sub.4 /F.sub.5                                                                     -0.95  -0.98  -0.96  -0.94  -0.97  -1.00  -0.90                        H.sub.1 /F.sub.1                                                                     -0.078 -0.085 -0.077 -0.080 -0.068 -0.068 -0.096                       β.sub.tw /Fw                                                                    2.53   2.51   3.11   2.66   2.15   2.49   2.99                         F.sub.1 /Fw                                                                          8.60   8.25   8.64   9.81   8.97   8.85   8.81                         ν.sub.4n /ν.sub.4p                                                             --     --     36.92  36.92  36.92  17.29  30.06                        Fw/φ.sub.1                                                                       0.150  0.54   0.172  0.144  0.175  0.171  0.171                                      EXAMPLE 8                                                                            EXAMPLE 9                                                                            EXAMPLE 10                                                                           EXAMPLE 11                                                                           EXAMPLE 12                                                                           EXAMPLE                      __________________________________________________________________________                                                     13                                  D.sub.4 w/Fw                                                                         2.87   2.99   2.45   3.05   2.51   2.54                                D.sub.2 w/D.sub.4 w                                                                  2.44   2.34   2.86   2.30   2.79   2.75                                D.sub.4 w/F.sub.4                                                                    -0.71  -0.67  -0.74  -0.75  -061   -0.63                               F.sub.1 /F.sub.2                                                                     -6.06  -6.08  -6.14  -6.17  -6.01  -6.01                               β.sub.5                                                                         0.16   0.19   0.13   0.24   0.18   0.16                                F.sub.2 /F.sub.4                                                                     0.37   0.34   0.45   0.37   0.37   0.37                                Z.sub.2 /Z                                                                           0.99   1.01   0.98   0.98   1.06   1.10                                 ##STR2##                                                                            -0.84  -0.34  -0.34  -0.34  -0.34  -0.34                               F.sub.3 /Fw                                                                          3.28   3.35   8.08   3.17   3.24   3.28                                F.sub.4 /Fw                                                                          -4.05  -4.47  -3.32  -4.06  4.08   -4.05                               F.sub.5 /Fw                                                                          3.78   3.89   3.50   3.44   3.89   3.82                                F.sub.4 /F.sub.5                                                                     -1.07  -1.15  -0.95  -1.18  -1.05  -1.06                               H.sub.1 /F.sub.1                                                                     -0.070 -0.073 -0.078 -0.080 -0.068 -0.070                              β.sub.tw /Fw                                                                    2.19   2.18   2.14   2.24   2.24   2.22                                F.sub.1 /Fw                                                                          9.14   9.18   9.26   9.31   9.11   9.11                                ν.sub.4n /ν.sub.4p                                                             36.92  36.92  36.92  36.92  36.92  36.92                               Fw/φ.sub.1                                                                       0.179  0.175  0.175  0.174  0.177  0.177                        __________________________________________________________________________

As described above, by arranging the movement of the movable lenses whenthe zooming is performed, a rear focusing zoom lens having a small sizeand realizing significant power variation can be provided. By causingthe fourth lens unit to be moved to form a concave locus facing theobject when the negative fourth lens unit is moved from the wide angleend to the telescopic end, the space can efficiently be used and thediameter of the first lens unit can be reduced. Furthermore, byappropriately setting the specifications of the lenses, a zoom lens,which has a small size and in which the aberration can be correctedsatisfactorily, can be provided.

Although the foregoing embodiment provides the zoom lenses having a highvariable power ratio of 20 times, zoom lenses of a type which does notpermit the foregoing high zooming ratio but which has smaller number oflenses to reduce the size thereof will now be described.

The rear focusing zoom lens according to this embodiment comprises afirst lens unit L1 having positive refracting power, a second lens unitL2 having negative refracting power, a third lens unit L3 havingpositive refracting power, a fourth lens unit L4 having negativerefracting power and a fifth lens unit L5 having positive refractingpower, the foregoing lenses being disposed in the foregoing sequentialorder when viewed from the object. The second lens unit is moved towardthe image surface to change the magnification from the wide angle end tothe telescopic end. The change in the image surface occurring due to themagnification change is corrected by moving the fourth lens unit.Furthermore, the fourth lens unit is moved along the optical axis toperform focusing. In addition, the third lens unit is composed of asingle positive lens. Note that the first, third and the fifth lensunits are always caused to remain stationary.

FIGS. 34(A)(1) to 39(B)(4) are aberration graphs according to Examples14 to 23 of the present invention to be described later. Referring tothe aberration graphs, graphs (A) show wide angle ends, and graphs (B)show telescopic-ends.

Also according to this embodiment, focusing is performed from aninfinite object to a near object, the fourth lens unit is movedrearwards (toward the image surface) as indicated by a straight line 4c.

The rear focusing zoom lens according to the present invention can berealized by satisfying the foregoing conditions. To further reduce thesize of the overall lens system, to prevent change in the aberrationoccurring due to magnification change performed when the magnificationis enlarged and to obtain excellent optical performance over the powervariation range, any of the following conditions must be satisfied.

(1--1) Assuming that the focal length of the i-th lens unit is Fi andthe focal length of the overall system at the wide angle end is F_(W),it is preferable that the following be satisfied:

    4<F1/F.sub.W <6.5                                          (1)

The foregoing expression is required to provide a zoom lens, themagnification of which is about 6 times while reducing the size andsimplifying the structure. The foregoing expression relates to theobject point with respect to the second lens unit, that is, to themagnification. To reduce the size of the overall system, it ispreferable that the second lens unit is disposed to cause the equalmagnification to intervene when the zooming is performed. If the equalmagnification is allowed to intervene, the locus of the zooming of thefourth lens unit is made to substantially reciprocate. Thus, significantpower variation can be performed with the most effectivespace-efficiency. Specifically, if the value is larger than the upperlimit, the focal length of the first lens unit is lengthened, the objectpoint with respect to the second lens unit which is the main lens unitfor varying the power is made remote, the image forming magnification ofthe second lens unit is decreased, and thus effective size reductioncannot easily be performed. What is worse, the distance from the firstlens unit to the second lens unit is excessively lengthened. Thus, thesize reduction cannot be performed.

If the value is smaller than the lower limit, the focal length of thefirst lens unit is shortened. If the horizontal imaging magnification ofthe second lens unit is intended to be maintained as described above,the refracting power of the second lens unit is strengthened andexcessive severe supporting accuracy is required.

(1-2) In the structure in which, from the object, a first lens unitfixed during zooming and having positive refracting power, a second lensunit movable during zooming and having negative refracting power, athird lens unit having positive refracting power, a fourth lens unit forcorrecting movement of the image point occurring when the zooming isperformed and having negative refracting power and a fifth lens unithaving positive refracting power, it is preferable that the rangefindingbe performed by the fourth lens unit. The fourth lens unit is movedtoward the image surface when rangefinding (focusing) of a near image isperformed. If the rangefinding is performed by moving the fourth lensunit as described above, decrease in the variable power ratio in thenear distance portion with respect to the paraxial variable power ratioas has been experienced with the conventional rear focusing method inwhich the positive lens unit performs the rangefinding can be prevented.Even if the variable power ratio is raised, a desired variable powerratio can be obtained in the short distance portion.

(1-3) It is preferable that the diaphragm SP relating to the reductionof the diameter of the front lens be located near the third lens unit atthe telescopic end. If the position of the diaphragm SP at the wideangle end is shifted more adjacent to the object than the position atthe telescopic end, the diameter of the front lens can effectively bereduced.

Generally, in the conventional zoom lens, the height of light, which isout of the axis and made incident upon the first lens unit, is highestat an intermediate zooming position adjacent to the wide angle endbecause the incident pupil is positioned deep (remote) from the firstlens unit. If the position, at which the incident light is highest, canbe set near the wide angle end by moving the incident pupil, that is,the diaphragm SP, the outer diameter of the first lens unit can bereduced most efficiently.

It is preferable that the diaphragm SP be fixed adjacent to the thirdlens unit during the zooming to constitute a desired mechanism. It ispreferable that the diaphragm SP be located adjacent to the third lensunit at this time. To reduce the diameter of the front lens, it ispreferable that the diaphragm SP be disposed in the third lens unit at aposition relatively adjacent to the object, preferably at a positionnearest the object.

(1-4) Assuming that the distance from the second lens unit to the thirdlens unit (in a case where the diaphragm is located at a positionadjacent to the third lens unit facing the object, to the surface of thediaphragm) at the wide angle end when the distance from an object isinfinite is D2_(W), and the distance from the fourth lens unit to thefifth lens unit is D4_(W), it is preferable that the following besatisfied:

    1.5<D2.sub.W /D4.sub.W <2.9                                (2)

The foregoing expression provides the space for moving the fourth lensunit for performing focusing that is lengthened at the telescopic end inrelation to the movable range for the second lens unit that is the mainlens unit for the power variation.

In the zoom lens employing the foregoing rear focusing method, theamount of movement of the focusing lens, at the telescopic end when thedistance from an object is short, is inevitably enlarged. In particular,the zooming ratio is raised in proportion to the zooming magnificationif the distance D2_(W), which enlarges both amount of movement of thefourth lens unit for correcting the image surface position of the sameand that for performing focusing, is long, that is, if the movable rangefor the zooming is large. Therefore, the amount of the fourth lens unitfor photographing a near image must be enlarged at the telescopic end.The foregoing expression indicates the foregoing correlation. If thevalue is larger than the upper limit, the close-up distance is madeexcessively remote at the telescopic end. If the value is smaller thanthe lower limit, the zoom lens having a poor zooming ratio is realized.

(1-5) It is preferable that the movement of the focusing lens satisfythe following expression:

    0.3<|D4.sub.W /F4|<0.5                   (3)

The foregoing expression provides appropriate refracting power of thefourth lens unit, which is the focusing lens, and the movable space forthe fourth lens unit. If the value is larger than the upper limit, aspace excessively longer than that required to perform a desired shortdistance photographing is undesirably set. Thus, the overall lengthcannot be shortened. If the value is smaller than the lower limit, aspace required to perform focusing cannot be maintained.

(1-6) It is preferable that the fourth lens unit satisfy the following:

    -2.6<F4/F.sub.W <-1.5                                      (4)

If the value is larger than the upper limit, the refracting power of thefourth lens unit is strengthened, and the Petzval sum is enlarged in arange of negative values. Thus, the image surface is correctedexcessively. If the value is smaller than the lower limit, the amount ofmovement of the fourth lens unit for providing a desired close-upphotographing distance is enlarged excessively. In this case, the sizeof the overall system is enlarged excessively.

(1-7) It is preferable that the second lens unit, which is the main lensunit for performing zooming, satisfy the following:

    -1.2>F2/F.sub.W <-0.8                                      (5)

If the value is larger than the upper limit, the refracting power of thesecond lens unit is strengthened and the Petzval sum is enlarged in arange of negative values. Thus, the image surface is excessivelycorrected. If the value is smaller than the lower limit, the amount ofmovement of the second lens unit for obtaining a desired variable powerratio is enlarged excessively. As a result, the diameter of the frontlens and the size of the overall system are enlarged excessively.

(1-8) It is preferable that light beams in a divergent system be madeincident on the third lens unit to be formed into a converged systembefore it is introduced into the fourth lens unit. That is, it ispreferable that the imaging magnification β3 be always negative valuesover the zooming range to reduce the size of the system.

    β3<0                                                  (6)

If the foregoing condition is satisfied, the fourth lens unit can bemoved efficiently, in particular, the fourth lens unit can be moved fromthe wide angle end to the telescopic end to form a convex locus facingthe image surface. More particularly, the fourth lens unit can be causedto substantially reciprocate. Thus, the size reduction can effectivelybe performed.

To reduce the size of the overall system and to effectively provide themovable space, the fourth lens unit must be caused to substantiallycompletely reciprocate or to form a convex locus facing the imagesurface, as described above. It is preferable that the imagingmagnification β3 of the third lens unit be a negative value and theabsolute value of the same be increased from the wide angle end towardthe telescopic end, followed by being decreased. Assuming that theparaxial horizontal magnifications of the third lens unit at the wideangle end and the telescopic end are β3W and β3T, the most excellentspace efficiency can be realized if the following is satisfied:

    β3W=β3T

Assuming that the maximum value of absolute value of the paraxialhorizontal magnification of the third lens unit from the wide angle endto the telescopic end is β3MAX, it is preferable that the following besatisfied:

    |β3MAX|>0.8

Further preferably, it is preferable that the value exceeds -1 toenlarge the magnification.

(1-9) Assuming that the ratio (β2T/β2w) of the imaging magnificationsfrom the wide angle end to the telescopic end is Z2, the focal length ofthe overall system at the telescopic end is Ft and the ratio (Ft/Fw) ofthe focal lengths at the wide angle end and the telescopic end of theoverall system is Z, it is preferable that the following be satisfied:

    0.68<Z2/Z<1.0                                              (8)

The foregoing expression indicates the ratio of the power variation ofthe second lens unit with respect to the overall power variation. If thevalue is larger than the upper limit, the magnification of any unit inthe lens system is decreased. Thus, the efficiency in varying the powerdeteriorates, and the size is enlarged excessively.

If the value is smaller than the lower limit, a plurality of lens unitsthat contribute to the power variation exit, thus arising a difficultyin controlling the lens units. Although excellent space efficiency isrealized if the fourth lens unit is allowed to completely reciprocate atthe time of performing zooming, the fourth lens unit is not sometimescompletely be returned to obtain a desired zooming ratio. At this time,also the fourth lens unit somewhat contributes to the power variation(|β3W|<|β3T|). It is preferable that the following be satisfied:

    0.68<Z2/Z<0.8

If the value is out of the foregoing range and thus the fourth lens unitconsiderably contributes to the power variation, the movable space forthe fourth lens unit is enlarged. Thus, the size of the overall systemis enlarged excessively.

(1-10) It is preferable that the third lens unit satisfy the followingexpression:

    1.5<F3/F.sub.W <2.0                                        (9)

The foregoing expression permits the magnification of the foregoingthird lens unit to be suitably provided. If the value is larger than theupper limit, the size reduction cannot be performed. If the value issmaller than the lower limit, the movable space required for the fourthlens unit to perform focusing cannot appropriately be provided.

If the value is out of the foregoing range, the fourth lens unit cannotefficiently be moved. In particular, the fourth lens unit cannot bemoved from the wide angle end to the telescopic end to form a convexfacing the image surface or cannot be allowed to reciprocatesubstantially completely. By composing the third lens unit by a singlelens, the size can be reduced. To reduce the spherical aberration, anaspherical surface may be provided for any surface of the third lensunit.

(1-11) It is preferable that the fourth lens unit, which is the focusinglens unit and is a lens unit for correcting the image point, has a smallsize and light weight in order to simplify the structure because thefourth lens unit is moved. To reduce the size and weight, it ispreferable that the fourth lens unit be composed of a single negativelens. To prevent change in the chromatic aberration at the time ofperforming focusing and zooming, it is preferable that the following besatisfied:

    20<ν4<50                                                (10)

where ν4 is the Abbe's number of the lens forming the fourth lens unit.It is preferable that the refraction of the single lens forming thefourth lens unit be within the following range:

    1.65<N4                                                    (11)

where N4 is the refractive index of the lens forming the fourth lensunit. If the value is out of the foregoing range, the curvature R issharpened excessively to realize the focal length F4 in the foregoingrange. In this case, the aberration deteriorates excessively.

When the fourth lens unit is composed of a single lens (or one blockformed by bonding two lenses), it is preferable that the fourth lensunit be double-concave lens. Assuming that the curvature radius facingthe object and that facing the image surface are R4a and R4b, it ispreferable that the following be satisfied:

    |R4a|/R4b>1.1                            (12)

As a result of the foregoing structure, the second principal point ofthe fourth lens unit is moved toward the fifth lens unit so that themovable range for the fourth lens unit is effectively enlarged.

To prevent generation of aberration so as to enlarge the movable range,it is preferable that the following be satisfied:

    1.4<|R4a|/R4b<3.0                        (12a)

(1-12) To cause the fifth lens unit, which is the final imaging lens, tohave a satisfactory aberration with respect to the focal length of theoverall system and to maintain appropriate back focus, it is preferablethat the following be satisfied:

    1.50<F5/F.sub.W <2.50                                      (13)

If the value is larger than the upper limit, the back focus islengthened and the size reduction cannot be achieved. If the value issmaller than the lower limit, the refracting power of the fifth lensunit is strengthened. As a result, undesirable spherical aberration orcomatic aberration are generated in, in particular, an intermediatezooming region. What is worse, the telecentric relationship is lost,causing the exit pupil to be shortened undesirably. Furthermore, ablock, such as a filter, cannot be inserted.

The structure according to the present invention has the arrangementthat the position of the exit pupil of the zoom lens is prevented frombeing set to be too short. Furthermore, diagonal beams from an objectare made incident to make a small angle so that the diameter of thefront lens is reduced.

If the range for the set power of the lens units from the diaphragmtoward the image surface is larger than the upper limit, the angle madeby the diagonal beams cannot be reduced. Thus, the diameter of the frontlens is enlarged undesirably. If the value is smaller than the lowerlimit, the exit pupil is shortened in a range of positive values. As aresult, telecentric beams with respect to the solid-image sensing devicecannot be maintained. What is worse, excessive change in the aberrationtakes place when zooming or focusing is performed.

If the value is included in the foregoing range, the size can be reducedsuch that excess movement does not take place when rangefinding (thefocusing) is performed. If the value is out of the foregoing range, thediameter of the front lens is enlarged and also the overall system isenlarged. As a result, an appropriate emission angle with respect to thesolid-image sensing device cannot be provided.

In particular, it is preferable that the position of the exit pupilsatisfy the following:

    -1.2<F4/F5<-0.80                                           (14)

If the value is larger than the upper limit, the refracting power of thefourth lens unit is strengthened and the focusing distance is changedexcessively. If the value is smaller than the lower limit, the amount ofmovement required when focusing is performed is enlarged and the sizereduction cannot be performed. Furthermore, the refracting power of thefifth lens unit, which is the fixed imaging lens, is strengthened and,therefore, the exit pupil is shortened excessively.

(1-13) To appropriately maintain the back focus of the lens whileappropriately providing the exit pupil of the optical system, thefollowing be satisfied assuming that the imaging magnification of thefifth lens unit is β5:

    |β5|<0.25                           (15)

If the value is out of the foregoing range, the back focus is enlarged,the lens system is enlarged and the exit pupil is shortened excessively.

(1-14) To cause the fifth lens unit, which is the final imaging lens, tohave an appropriate aberration with respect to the focal length of theoverall system and to maintain an appropriate back focus, it ispreferable that the following be satisfied:

    0.68<F5/√F.sub.W ×Ft<0.92                     (16)

If the value is larger than the upper limit, the back focus islengthened and, therefore, the size is enlarged excessively. If thevalue is smaller than the lower limit, the refracting power of the fifthlens unit is strengthened to cause, in particular, spherical aberrationand comatic aberration to be generated particularly in an intermediatezooming region. Furthermore, the telecentric relationship is lost andthe exit pupil is shortened excessively. (1-15) To prevent change in theposition of the exit pupil with respect to the solid-image sensingdevice while maintaining an appropriate back focus, it is preferablethat the imaging magnification β4 of the fourth lens unit always has thesame sign during zooming. If the imaging magnification β4 is changedwith its sign also changed during zooming, the incidental angle on thefixed fifth lens unit, which is the final imaging lens unit, is changedconsiderably. The incidental angle of the peripheral beams on thesolid-image sensing device (a CCD or the like) is changed considerablywhen the zooming is performed. Thus, an excessive deviation from thetelecentric formed image takes place, thus causing shading to begenerated undesirably.

To prevent shading at the wide angle end and the telescopic end at whichthe exit pupil is shortened in a range of positive values and tomaintain an appropriate back focus, it is preferable that the followingbe satisfied:

    β4W×β4T>0                                  (17)

where the paraxial horizontal magnification of the fourth lens unit atthe wide angle end and that of the telescopic end when the distance froman object is infinite are β4W and β4T, respectively.

(1-16) To reduce the size of the overall system, it is preferable thatthe following be satisfied:

    1.5<Bf.sub.W /F.sub.W <2.2                                 (18)

where Bf_(W) is the back focus (except an element, such as a glass blockor a filter, represented by G in the embodiment) at the wide angle endwhen the distance from an object is infinite. The foregoing expressionis required to reduce effectively the size of the overall system. If thevalue is smaller than the lower limit, a block, such as a filter, cannotbe inserted, and the exit pupil is shortened. As a result, the formingof an image onto the image pickup device is undesirably out of thetelecentric system. If the value is larger than the upper limit, desiredsize reduction cannot be performed.

(1-17) It is preferable that the first lens unit consists of a pluralityof lenses including at least one negative lens; the second lens unitconsists of a plurality of lenses including at least one positive lens;the third lens unit consists of one positive lens; the fourth lens unitconsists of at least one negative lens; and the fifth lens unit consistsof a plurality of lens including at least one negative lens.

In particular, it is preferable that the first lens unit comprises threelenses consisting of, when viewed from the object, a negative meniscuslens having a convex facing the object, a positive lens having a convexfacing the object, and a positive lens having a convex facing theobject. It is preferable that the second lens unit comprises, whenviewed from the object, a negative meniscus lens having a convex facingthe object, a double-convex lens and a positive lens.

An aspherical surface may be provided for any of the lens surfaces ofthe third lens unit. The aspherical surface may be disposed either onthe object side or on the image surface side. In particular, it ispreferable that the aspherical surface be disposed on the object side tocorrect the spherical aberration.

The fourth lens unit moves to correct change of the second lens unitoccurring during zooming or correct change in the image point of thesecond lens unit occurring when the second lens unit is moved, as wellas to perform focusing. In particular, the fourth lens unit may be, inplace of being composed of the single lens, composed of two lensesconsisting of a positive lens and a negative lens in order to preventchange in the chromatic aberration. The positive lens and the negativelens may be formed into a single block by bonding. Assuming that theAbbe's numbers of the positive lens and the negative lens are ν4p andν4n respectively, it is preferable that the following be satisfied:

    ν4p<ν4n

If the value is out of the foregoing range, the chromatic aberration ischanged considerably dependently on the distance.

By composing the fourth lens unit by bonding the positive lens and thenegative lens, the fourth lens unit, which is moved when zooming isperformed and when focusing is performed, can be formed into a singleblock. Thus, the structure can be simplified and the fourth lens unitcan easily be supported like the foregoing single lens structure.

Assuming that the Abbe's numbers of the positive lens and the negativelens constituting the fourth lens unit are ν4p and ν4n, it is preferablethat the following be satisfied:

    ν4n-ν4p>15

To effectively change the chromatic aberration during the zooming, it ispreferable that the following be satisfied:

    ν4n-ν4p>30

In a case where the lenses composing the fourth lens unit are unifiedinto one block, in particular, in a case where the same is composed ofthe positive lens and the negative lens that are bonded to each other orin a case where the same is composed of one negative lens, it ispreferable that the concave of the block faces the object and that theconcave of the same facing the image surface faces the image.

The fifth lens unit is a positive lens unit that is fixed during zoomingand that easily encounters spherical aberration. To prevent thespherical aberration, it is preferable that the fifth lens unit becomposed of at least one positive lens and at least one negative lens.If the fifth lens unit is composed of one positive lens and one negativelens, it is preferable that an aspherical surface be provided for eitherlens surface to satisfactorily correct the spherical aberration. Tocorrect the chromatic aberration, a bonded lens unit may be employed.

If the fifth lens unit is composed of one positive lens and one negativelens, it is preferable that a structure be employed in which a negativemeniscus lens having a convex facing an object and a double-convex lensare disposed in this sequential order when viewed from the object. As aresult, the second principal point is located at a position facing theimage surface, and the back focus can effectively be maintained. The twolenses may be bonded to each other so as to be unified into one block.As a result, the lens unit can easily be supported and the structure canbe simplified.

(1-18) To widen the angle, it is an important fact to shorten, at thewide angle end, the distance e1 between the principal point of the firstlens unit and that of the second lens unit. To achieve this, it ispreferable that the shape of the second lens unit be as follows;

To shorten, at the wide angle end, the distance e1 between the twoprincipal points, it is preferable that the (first) principal point ofthe second lens unit facing the object be positioned adjacent to theobject for the purpose of widen the angle. Specifically, it ispreferable that configuration be arranged in such a manner that, whenviewed from the object, there are disposed a negative meniscus lens L21having a convex facing the object, a double-concave negative lens L21and a positive lens L23 while causing an air gap to intervene. Theforegoing air gap causes the principal point of the second lens unitfacing the object to be shifted toward the first lens unit. Thus, thedistance e1 between the two principal points can easily be shortened inthe wide angle portion, and, therefore, the angle can effectively bewidened. A negative lens L24 may be disposed adjacent to the positivelens L23 at a position adjacent to the image surface. The negative lensL24 causes the principal point of the second lens unit facing the objectto be shifted toward the first lens unit. Furthermore, change in thechromatic aberration occurring due to the zooming can be prevented.

(1-19) A small size and wide angle zoom lens can be realized if thefollowing is satisfied assuming that the outer diameter of the lenspositioned nearest the object is φ1 and the focal length of the overallsystem at the wide angle end is F_(W) :

    0.10<F.sub.W /φ1<0.20                                  (19)

Generally, if the focal length F_(W) at the wide angle end is shortened,the diameter φ1 of the front lens is enlarged. If the focal length F_(W)at the wide angle end is lengthened on the contrary, the diameter φ1 ofthe front lens is reduced though it depends upon the brightness at thetelescopic end of the lens. Expression (19) indicates an appropriaterelationship between the diameter of the front lens and the focal lengthat the wide angle end to realize a small size and wide angle zoom lens.If the ratio is out of the foregoing range, the small size and wideangle zoom lens cannot be provided. Specifically, if the value is largerthan the upper limit, a zoom lens of a telescopic type is realized. Ifthe value is smaller than the lower limit, a zoom lens having a largesize is realized.

The diameter φ1 of the front lens is substantially the opticallyeffective diameter of the first lens unit and is an actual outerdiameter of the lens. The diameter φ1 of the front lens is larger thanthe optically effective diameter of the first lens unit by 0% to 6%.

Then, examples of the present invention will now be described. In theexamples, Ri is the curvature radius of the i-th lens surface counted ina direction from an object, Di is the thickness of the lens and the airgap counted in the direction from the object, and Ni and νi arerefracting power and Abbe's number of the i-lens counted in thedirection from the object.

In the examples, the second lens surface counted from the final lens isa glass member, such as a face plate. The relationships among eachconditional expression and various values in the examples are shown inTable 1. The aspherical surface is expressed as follows when X-axisstands for the direction of the optical axis, H-axis stands for thedirection perpendicular to the optical axis, a direction in which lighttravels is indicated by positive value, R is paraxial curvature radiusand K, A, B, C, D and E are aspherical surface coefficients: ##EQU2##

EXAMPLE

    __________________________________________________________________________    f = 1˜5.98        Fno = 1.85˜2.17          2ω =             61.7°˜11.4°                                               __________________________________________________________________________    R1  = 5.858 D1  = 0.28 N1  = 1.846659                                                                         ν1  = 23.8                                 R2  = 3.628 D2  = 1.26 N2  = 1.603112                                                                         ν2  = 60.7                                 R3  = -478.413                                                                            D3  = 0.04                                                        R4  = 2.985 D4  = 0.73 N3  = 1.603112                                                                         ν3  = 60.7                                 R5  = 7.503 D5  = VARIABLE                                                    R6  = 7.219 D6  = 0.12 N4  = 1.806098                                                                         ν4  = 41.0                                 R7  = 0.994 D7  = 0.44                                                        R8  = -2.897                                                                              D8  = 0.12 N5  = 1.806098                                                                         ν5  = 41.0                                 R9  = 2.028 D9  = 0.21                                                        R10 = 2.279 D10 = 0.44 N6  = 1.846659                                                                         ν6  = 23.8                                 R11 = 36.548                                                                              D11 = VARIABLE                                                    R12 = DIAPHRAGM                                                                           D12 = 0.27                                                        R13 = 2.069 D13 = 0.73 N7  = 1.583126                                                                         ν7  = 59.4                                 R14 = -1.988                                                                              D14 = VARIABLE                                                    R15 = -4.254                                                                              D15 = 0.20 N8  = 1.806098                                                                         ν8  = 41.0                                 R16 = 2.642 D16 = VARIABLE                                                    R17 = 3.390 D17 = 0.49 N9  = 1.516330                                                                         ν9  = 64.2                                 R18 = -2.961                                                                              D18 = 0.04                                                        R19 = 2.046 D19 = 0.15 N10 = 1.846659                                                                         ν10 = 23.8                                 R20 = 1.158 D20 = 0.61 N11 = 1.487490                                                                         ν11 = 70.2                                 R21 = -6.436                                                                              D21 = 0.49                                                        R22 = ∞                                                                             D22 = 1.05 N12 = 1.516330                                                                         ν12 = 64.2                                 R23 = ∞                                                                 __________________________________________________________________________               FOCAL LENGTH                                                       VARIABLE DISTANCE                                                                        1.00        3.84                                                                             5.98                                                __________________________________________________________________________    D5         0.20        1.79                                                                             2.24                                                D11        2.35        0.76                                                                             0.30                                                D14        0.29        0.78                                                                             0.61                                                D16        0.86        0.37                                                                             0.54                                                __________________________________________________________________________    ASPHERICAL COEFFICIENT                                                            K      B      C      D      E                                             __________________________________________________________________________    13-TH                                                                             -1.659e + 00                                                                         -4.121e - 02                                                                         -1.083e - 02                                                                         1.125e - 02                                                                          -1.238e - 02                                  SIDE                                                                          __________________________________________________________________________

EXAMPLE

    __________________________________________________________________________    f = 1˜6.00        Fno = 1.85˜2.08          2ω =             61.7°˜11.4°                                               __________________________________________________________________________    R1  = 5.850 D1  = 0.28 N1  = 1.846659                                                                         ν1  = 23.8                                 R2  = 3.656 D2  = 1.36 N2  = 1.603112                                                                         ν2  = 60.7                                 R3  = -165.992                                                                            D3  = 0.04                                                        R4  = 3.005 D4  = 0.74 N3  = 1.603112                                                                         ν3  = 60.7                                 R5  = 7.300 D5  = VARIABLE                                                    R6  = 7.031 D6  = 0.12 N4  = 1.806098                                                                         ν4  = 41.0                                 R7  = 0.994 D7  = 0.46                                                        R8  = -2.861                                                                              D8  = 0.12 N5  = 1.806098                                                                         ν5  = 41.0                                 R9  = 2.026 D9  = 0.21                                                        R10 = 2.259 D10 = 0.44 N6  = 1.846659                                                                         ν6  = 23.8                                 R11 = 38.408                                                                              D11 = VARIABLE                                                    R12 = DIAPHRAGM                                                                           D12 = 0.27                                                        R13 = 2.089 D13 = 0.73 N7  = 1.583126                                                                         ν7  = 59.4                                 R14 = -1.995                                                                              D14 = VARIABLE                                                    R15 = -4.166                                                                              D15 = 0.20 N8  = 1.806098                                                                         ν8  = 41.0                                 R16 = 2.611 D16 = VARIABLE                                                    R17 = 3.368 D17 = 0.49 N9  = 1.516330                                                                         ν9  = 64.2                                 R18 = -2.981                                                                              D18 = 0.04                                                        R19 = 2.034 D19 = 0.15 N10 = 1.846659                                                                         ν10 = 23.8                                 R20 = 1.160 D20 = 0.61 N11 = 1.487490                                                                         ν11 = 70.2                                 R21 = -5.416                                                                              D21 = 0.49                                                        R22 = ∞                                                                             D22 = 1.05 N12 = 1.516330                                                                         ν12 = 64.2                                 R23 = ∞                                                                 __________________________________________________________________________               FOCAL LENGTH                                                       VARIABLE DISTANCE                                                                        1.00        3.87                                                                             6.00                                                __________________________________________________________________________    D5         0.20        1.79                                                                             2.24                                                D11        2.35        0.76                                                                             0.30                                                D14        0.33        0.80                                                                             0.59                                                D16        0.84        0.37                                                                             0.57                                                __________________________________________________________________________    ASPHERICAL COEFFICIENT                                                            K      B      C      D      E                                             __________________________________________________________________________    13-TH                                                                             -1.659e + 00                                                                         -4.141e - 02                                                                         -1.082e - 02                                                                         1.125e - 02                                                                          -1.238e - 02                                  SIDE                                                                          __________________________________________________________________________

EXAMPLE

    __________________________________________________________________________    f = 1˜6.00        Fno = 1.85˜2.04          2ω =             61.7°˜11.4°                                               __________________________________________________________________________    R1  = 6.215 D1  = 0.28 N1  = 1.846659                                                                         ν1  = 23.8                                 R2  = 3.900 D2  = 1.42 N2  = 1.603112                                                                         ν2  = 60.7                                 R3  = -111.124                                                                            D3  = 0.04                                                        R4  = 2.879 D4  = 0.84 N3  = 1.603112                                                                         ν3  = 60.7                                 R5  = 6.193 D5  = VARIABLE                                                    R6  = 5.998 D6  = 0.12 N4  = 1.806098                                                                         ν4  = 41.0                                 R7  = 0.991 D7  = 0.46                                                        R8  = -2.633                                                                              D8  = 0.12 N5  = 1.806098                                                                         ν5  = 41.0                                 R9  = 2.041 D9  = 0.21                                                        R10 = 2.343 D10 = 0.44 N6  = 1.846659                                                                         ν6  = 23.8                                 R11 = 42.205                                                                              D11 = VARIABLE                                                    R12 = DIAPHRAGM                                                                           D12 = 0.27                                                        R13 = 2.270 D13 = 0.73 N7  = 1.583126                                                                         ν7  = 59.4                                 R14 = -1.862                                                                              D14 = VARIABLE                                                    R15 = -6.532                                                                              D15 = 0.20 N8  = 1.806098                                                                         ν8  = 41.0                                 R16 = 2.829 D16 = VARIABLE                                                    R17 = 1.873 D17 = 0.15 N9  = 1.846659                                                                         ν9  = 23.8                                 R18 = 1.085 D18 = 0.85 N10 = 1.583126                                                                         ν10 = 59.4                                 R19 = -2.044                                                                              D19 = 0.49                                                        R20 = ∞                                                                             D20 = 1.05 N11 = 1.516330                                                                         ν11 = 64.2                                 R21 = ∞                                                                 __________________________________________________________________________               FOCAL LENGTH                                                       VARIABLE DISTANCE                                                                        1.00        3.76                                                                             6.00                                                __________________________________________________________________________    D5         0.20        1.79                                                                             2.24                                                D11        2.35        0.76                                                                             0.30                                                D14        0.31        0.81                                                                             0.63                                                D16        1.14        0.64                                                                             0.82                                                __________________________________________________________________________    ASPHERICAL COEFFICIENT                                                            K      B      C      D      E                                             __________________________________________________________________________    13-TH                                                                             -1.654e + 00                                                                         -5.235e - 02                                                                         -1.093e - 02                                                                           1.205e - 02                                                                        -1.465e - 02                                  SIDE                                                                          19-TH                                                                             -5.996e - 01                                                                           2.686e - 02                                                                        -6.032e - 03                                                                         -1.379e - 02                                                                         -1.860e - 03                                  SIDE                                                                          __________________________________________________________________________

EXAMPLE

    __________________________________________________________________________    f = 1˜5.98        Fno = 1.85˜2.10          2ω =             61.7°˜11.4°                                               __________________________________________________________________________    R1  = 6.182 D1  = 0.28 N1  = 1.846659                                                                         ν1  = 23.8                                 R2  = 4.042 D2  = 1.72 N2  = 1.603112                                                                         ν2  = 60.7                                 R3  = -391.245                                                                            D3  = 0.04                                                        R4  = 2.986 D4  = 0.98 N3  = 1.603112                                                                         ν3  = 60.7                                 R5  = 6.235 D5  = VARIABLE                                                    R6  = 6.112 D6  = 0.12 N4  = 1.806098                                                                         ν4  = 41.0                                 R7  = 0.975 D7  = 0.46                                                        R8  = -2.510                                                                              D8  = 0.12 N5  = 1806098                                                                          ν5  = 41.0                                 R9  = 2.066 D9  = 0.21                                                        R10 = 2.259 D10 = 0.44 N6  = 1.846659                                                                         ν6  = 23.8                                 R11 = 26.760                                                                              D11 = VARIABLE                                                    R12 = DIAPHRAGM                                                                           D12 = 0.21                                                        R13 = 2.257 D13 = 0.73 N7  = 1.583126                                                                         ν7  = 59.4                                 R14 = -1.874                                                                              D14 = VARIABLE                                                    R15 = -6.651                                                                              D15 = 0.20 N8  = 1.806098                                                                         ν8  = 41.0                                 R16 = 2.890 D16 = VARIABLE                                                    R17 = 1.825 D17 = 0.15 N9  = 1.846659                                                                         ν9  = 23.8                                 R18 = 1.057 D18 = 0.85 N10 = 1.583126                                                                         ν10 = 59.4                                 R19 = -1.989                                                                              D19 = 0.49                                                        R20 = ∞                                                                             D20 = 1.05 N11 = 1.516330                                                                         ν11 = 64.2                                 R21 = ∞                                                                 __________________________________________________________________________               FOCAL LENGTH                                                       VARIABLE DISTANCE                                                                        1.00        3.74                                                                             5.98                                                __________________________________________________________________________    D5         0.20        1.76                                                                             2.20                                                D11        2.31        0.75                                                                             0.30                                                D14        0.30        0.86                                                                             0.72                                                D16        1.02        0.46                                                                             0.60                                                __________________________________________________________________________    ASPHERICAL COEFFICIENT                                                            K      B      C      D      E                                             __________________________________________________________________________    13-TH                                                                             -1.654e + 00                                                                         -5.302e - 02                                                                         -1.100e - 02                                                                           1.205e - 02                                                                        -1.465e - 02                                  SIDE                                                                          19-TH                                                                             -5.996e - 01                                                                           2.852e - 02                                                                        -5.979e - 03                                                                         -1.379e - 02                                                                         -1.860e - 03                                  SIDE                                                                          __________________________________________________________________________

EXAMPLE

    __________________________________________________________________________    f = 1˜5.97        Fno = 1.85˜2.13          2ω =             61.7°˜11.4°                                               __________________________________________________________________________    R1  = 6.454 D1  = 0.28 N1  = 1.846659                                                                         ν1  = 23.8                                 R2  = 4.213 D2  = 2.16 N2  = 1.603112                                                                         ν2  = 60.7                                 R3  = 273.515                                                                             D3  = 0.04                                                        R4  = 2.979 D4  = 1.01 N3  = 1.603112                                                                         ν3  = 60.7                                 R5  = 6.298 D5  = VARIABLE                                                    R6  = 5.324 D6  = 0.12 N4  = 1.772499                                                                         ν4  = 49.6                                 R7  = 0.927 D7  = 0.46                                                        R8  = -2.452                                                                              D8  = 0.12 N5  = 1.806098                                                                         ν5  = 41.0                                 R9  = 2.041 D9  = 0.21                                                        R10 = 2.255 D10 = 0.44 N6  = 1.846659                                                                         ν6  = 23.8                                 R11 = 33.368                                                                              D11 = VARIABLE                                                    R12 = DIAPHRAGM                                                                           D12 = 0.27                                                        R13 = 2.269 D13 = 0.73 N7  = 1.583126                                                                         ν7  = 59.4                                 R14 = -1.858                                                                              D14 = VARIABLE                                                    R15 = -7.346                                                                              D15 = 0.20 N8  = 1.846659                                                                         ν8  = 23.8                                 R16 = 3.047 D16 = VARIABLE                                                    R17 = 1.894 D17 = 0.15 N9  = 1.846659                                                                         ν9  = 23.8                                 R18 = 1.273 D18 = 0.02                                                        R19 = 1.353 D19 = 0.85 N10 = 1.583126                                                                         ν10 = 59.4                                 R20 = -1.990                                                                              D20 = 0.49                                                        R21 = ∞                                                                             D21 = 1.05 N11 = 1.516330                                                                         ν11 = 64.2                                 R22 = ∞                                                                 __________________________________________________________________________               FOCAL LENGTH                                                       VARIABLE DISTANCE                                                                        1.00        3.71                                                                             5.97                                                __________________________________________________________________________    D5         0.20        1.79                                                                             2.23                                                D11        2.34        0.75                                                                             0.30                                                D14        0.29        0.86                                                                             0.76                                                D16        1.06        0.49                                                                             0.59                                                __________________________________________________________________________    ASPHERICAL COEFFICIENT                                                            K      B      C      D      E                                             __________________________________________________________________________    13-TH                                                                             -1.654e + 00                                                                         -5.606e - 02                                                                         -1.108e - 02                                                                           1.205e - 02                                                                        -1.465e - 02                                  SIDE                                                                          20-TH                                                                             -5.997e - 01                                                                         3.118e - 02                                                                          -5.914e - 03                                                                         -1.378e - 02                                                                         -1.860e - 03                                  SIDE                                                                          __________________________________________________________________________

EXAMPLE

    __________________________________________________________________________    f = 1˜5.97        Fno = 1.85˜2.15          2ω =             61.7°˜11.4°                                               __________________________________________________________________________    R1  = 6.447 D1  = 0.28 N1  = 1.846659                                                                         ν1  = 23.8                                 R2  = 4.213 D2  = 2.16 N2  = 1.603112                                                                         ν2  = 60.7                                 R3  = 283.391                                                                             D3  = 0.04                                                        R4  = 3.153 D4  = 1.01 N3  = 1.638539                                                                         ν3  = 55.4                                 R5  = 6.691 D5  = VARIABLE                                                    R6  = 5.555 D6  = 0.12 N4  = 1.806098                                                                         ν4  = 41.0                                 R7  = 0.968 D7  = 0.46                                                        R8  = -2.452                                                                              D8  = 0.12 N5  = 1.806098                                                                         ν5  = 41.0                                 R9  = 2.041 D9  = 0.21                                                        R10 = 2.255 D10 = 0.44 N6  = 1.846659                                                                         ν6  = 23.8                                 R11 = 33.376                                                                              D11 = VARIABLE                                                    R12 = DIAPHRAGM                                                                           D12 = 0.27                                                        R13 = 2.269 D13 = 0.73 N7  = 1.583126                                                                         ν7  = 59.4                                 R14 = -1.858                                                                              D14 = VARIABLE                                                    R15 = -7.346                                                                              D15 = 0.20 N8  = 1.846659                                                                         ν8  = 23.8                                 R16 = 3.046 D16 = VARIABLE                                                    R17 = 1.894 D17 = 0.15 N9  = 1.846659                                                                         ν9  = 23.8                                 R18 = 1.273 D18 = 0.02                                                        R19 = 1.353 D19 = 0.85 N10 = 1.583126                                                                         ν10 = 59.4                                 R20 = -2.013                                                                              D20 = 0.49                                                        R21 = ∞                                                                             D21 = 1.05 N11 = 1.516330                                                                         ν11 = 64.2                                 R22 = ∞                                                                 __________________________________________________________________________               FOCAL LENGTH                                                       VARIABLE DISTANCE                                                                        1.00        3.71                                                                             5.97                                                __________________________________________________________________________    D5         0.20        1.79                                                                             2.24                                                D11        2.35        0.75                                                                             0.30                                                D14        0.29        0.87                                                                             0.78                                                D16        1.06        0.48                                                                             0.57                                                __________________________________________________________________________    ASPHERICAL COEFFICIENT                                                            K      B      C      D      E                                             __________________________________________________________________________    13-TH                                                                             -1.654e + 00                                                                         -5.606e - 02                                                                         -1.108e - 02                                                                           1.205e - 02                                                                        -1.465e - 02                                  SIDE                                                                          20-TH                                                                             -5.997e - 01                                                                           3.118e - 02                                                                        -5.914e - 03                                                                         -1.378e - 02                                                                         -1.860e - 03                                  SIDE                                                                          __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________    CONDITIONAL                                                                              EXAMPLES                                                           EXPRESSION 14   15  16  17  18  19                                            __________________________________________________________________________    (1) F1/Fw  4.98 5.00                                                                              5.11                                                                              5.30                                                                              5.52                                                                              5.51                                          (2) D2w/D4w                                                                              2.74 2.80                                                                              2.07                                                                              2.27                                                                              2.21                                                                              2.21                                          (3) |D4w/F4|                                                           0.43 0.43                                                                              0.47                                                                              0.41                                                                              0.42                                                                              0.42                                          (4) F4/Fw  -2.00                                                                              -1.97                                                                             -2.43                                                                             -2.48                                                                             -2.52                                                                             -2.52                                         (5) F2/Fw  -0.99                                                                              -0.99                                                                             -0.97                                                                             -0.94                                                                             -0.95                                                                             -0.95                                         (6) β3W                                                                             -0.57                                                                              -0.57                                                                             -0.58                                                                             -0.59                                                                             -0.59                                                                             -0.58                                             β3T                                                                             -0.73                                                                              -0.71                                                                             -0.75                                                                             -0.81                                                                             -0.84                                                                             -0.84                                         (7) |β3MAX|                                                       0.82 0.81                                                                              0.84                                                                              0.90                                                                              0.90                                                                              0.89                                          (8) Z2/Z   0.75 0.79                                                                              0.78                                                                              0.73                                                                              0.70                                                                              0.70                                          (9) F3/Fw  1.86 1.87                                                                              1.88                                                                              1.88                                                                              1.87                                                                              1.87                                          (10)                                                                              ν4  41   41  41  41  23.8                                                                              23.8                                          (11)                                                                              N4     1.806                                                                              1.806                                                                             1.806                                                                             1.806                                                                             1.847                                                                             1.847                                         (12)                                                                              |R4a|/R4b                                                          1.61 1.60                                                                              2.31                                                                              2.30                                                                              2.41                                                                              2.41                                          (13)                                                                              F5/Fw  2.06 2.00                                                                              2.18                                                                              2.12                                                                              2.11                                                                              2.13                                          (14)                                                                              F4/F5  -0.97                                                                              -0.98                                                                             -1.11                                                                             -1.17                                                                             -1.19                                                                             -1.18                                         (15)                                                                              |β5|                                                          -0.15                                                                              -0.16                                                                             0.03                                                                              0.03                                                                              0.05                                                                              0.05                                          (16)                                                                               ##STR3##                                                                            0.84 0.82                                                                              0.89                                                                              0.87                                                                              -0.87                                                                             0.87                                          (17)                                                                              β4W                                                                             -6.27                                                                              -5.85                                                                             33.09                                                                             30.98                                                                             18.97                                                                             18.22                                             β4T                                                                             -6.42                                                                              -5.99                                                                             32.96                                                                             30.81                                                                             18.78                                                                             18.02                                         (18)                                                                              Bfw/Fw 1.76 1.73                                                                              1.78                                                                              1.73                                                                              1.7 1.71                                          (19)                                                                              Fw/φ1                                                                            0.18 0.18                                                                              0.17                                                                              0.15                                                                              0.13                                                                              0.13                                          __________________________________________________________________________

In the foregoing embodiments, the zoom lens has been described in such amanner that a great importance is placed on the third lens unit. In thedescription below, the zoom lens according to the present invention willnow be described in such a manner that a great importance is placed onthe fourth lens unit.

FIGS. 40 to 47 are cross sectional views showing lenses according toExamples 20 to 27 according to the present invention to be describedlater. FIGS. 48(A)(1) to 55(B)(4) are graphs showing various aberrationsof the examples 20 to 27. In each graph showing the aberration, graph(A) shows the wide angle end and (B) shows the telescopic end.

Referring to the drawings, symbol L1 represents a first unit (a firstlens unit) having positive refracting power, L2 represents a second unit(a second lens unit) having negative refracting power, L3 represents athird unit (a third lens unit) having positive refracting power, L4represents a fourth unit (a fourth lens unit) having negative refractingpower, and L5 represents a fifth unit (a fifth lens unit) havingpositive refracting power. Symbol SP represents a diaphragm that islocated in front of the third unit L3. Symbol IP represents an imagesurface.

When the power is varied from the wide angle end to the telescopic end,the second unit L2 is moved toward the image surface as indicated by anarrow. Furthermore, change in the image surface occurring when the poweris varied is corrected by moving the fourth unit in such a manner that aconvex locus facing the image surface is formed. The zoom lens accordingto this embodiment employs a rear focusing method in which focusing isperformed by moving the fourth unit along the optical axis.

Continuous line curve 4a and dashed line curve 4b indicate loci ofmovements for correcting change in the image surface occurring when thepower is changed from the wide angle end to the telescopic end in a casewhere an infinite object and a near object are focused. The first,third, and the fifth units are fixed when the power is varied and whenfocusing is performed.

In this embodiment, the fourth unit is moved to correct the change inthe image surface occurring when the power is varied. Furthermore, thefourth unit is moved to perform focusing. As indicated by the curves 4aand 4b, the fourth unit is moved to form a convex locus facing the imagesurface when the power is varied from the wide angle end to thetelescopic end. As a result, air between the fourth unit and the fifthunit can be used effectively and the overall length of the lens can beshortened effectively.

In this embodiment, when focusing is performed at the telescopic endfrom an infinite object to a near object, the fourth unit is movedrearwards as indicated by a straight line 4c.

In this embodiment, the fourth unit is composed of a single lens block,such as a single lens or bonded lens block, as employed in Examples 20to 27 to reduce the size and the weight of the fourth unit. As a result,the fourth unit is permitted to perform power variation and focusing athigh speed.

The rear focusing zoom lens according to the present invention ischaracterized as described above. Moreover, it is preferable that atleast one of the following conditions be satisfied to satisfactorilyform the structure.

(2-1) The fourth lens unit is composed of a negative lens having twolens surfaces each of which is concave. Assuming that the curvatureradii of the surfaces of the negative lens facing the object and theimage surface are R4a and R4b, the following must be satisfied:

    1.1<|R4a|/R4b

As a result, the second principal point of the fourth lens unit isshifted toward the fifth lens-unit, and, therefore, the movable rangefor the fourth lens unit can be enlarged. Thus, the size reduction canbe performed effectively.

(2-2) Generally, a so-called front lens focusing method for the zoomlens has been known in which rangefinding (focusing) is performed bymoving the first lens unit along the optical axis. The foregoing method,however, encounters a fact that the diameter of the front lens isinevitably enlarged because light beams are, at the wide angle portion,maintained in. the peripheral image surface when close-up photographingoperation is performed. Accordingly, the foregoing focusing methodencounters a difficulty in reducing the size that is one of the objectsof the present invention.

It is preferable that the first lens unit, having the largest diameterin the system, be fixed during zooming to simplify the structure. As aresult, it is preferable that lens units disposed to follow the thirdlens unit, in particular, the fourth lens unit performs focusing in acase where size reduction is the object. Furthermore, it is preferablethat also the third lens unit having a diaphragm unit be fixed tosimplify the mechanism. If the third lens unit having the diaphragm unitis composed of a single lens, the mechanism in the vicinity of thediaphragm unit, can be reduced in size and assembling can be performedsimply. Furthermore, the movable space for the fourth lens unit caneasily be maintained.

It is preferable that the fourth lens unit be moved from the wide angleend to the intermediate zooming region toward the image surface. When azoom lens capable of considerably varying the power is constituted, thefourth lens unit is moved from the wide angle end to the telescopic endto form a locus having convex facing the image surface, and moreparticularly, the fourth lens unit is caused to substantiallyreciprocate. In this case, a significant space saving efficiency can beobtained and, therefore, a zoom lens having a small size and capable ofsignificantly varying the power can be realized. It is preferable thatthe horizontal magnification of the second lens unit be changed duringzooming in such a manner that equal magnification (-1) is allowed tointervene.

The physical meaning of reduction in the diameter of the front lenspermitted in the foregoing structure will now be described.

Hitherto, when the angle is intended to be widened, the height of lightbeams positioned outside the axis and made incident upon the first lensunit is increased at the intermediate zooming position adjacent to thewide angle end. As a result, the effective diameter of the first lensunit is enlarged. To prevent the enlargement of the diameter of thefront lens, it is preferable that angle θ of light beams positionedoutside the axis and made incident upon the pupil (the diaphragm) fromthe object at the foregoing intermediate zooming position be set to asmall angle. In order to achieve this, it is preferable that the fourthlens unit be moved from the wide angle end to the intermediate zoomingregion toward the image surface.

As a result, the focal length that determines the diameter of the frontlens is considerably shifted toward the telescopic portion. Thus, thediameter can be reduced. It is effective that the exit pupil of the lensbe located in a range of positive values (in a range from the imagesurface opposing the object) over the entire zooming region.

It is preferable for the present invention that the exit pupil belocated from the image surface in the region of positive values(opposing the object). It is further preferable that the exit pupil bein a positive values at least at the wide angle end that contributes todetermining the diameter of the front lens.

(2-3) Assuming that the focal length of each of the i-th lens unit is Fiand the focal lengths of the overall system at the wide angle end andthe telescopic end are F_(W) and Ft, it is preferable that the followingbe satisfied:

    4.0<F1/F.sub.W <12

The foregoing conditions are required to provide a zoom lens having asmall size and simple structure and capable of varying the power by 6times or more according to the present invention.

The foregoing expression relates to the object point with respect to thesecond lens unit, that is, to the magnification. To reduce the size ofthe overall system, it is preferable that the second lens unit performszooming to allow equal magnification to intervene. If the equalmagnification is allowed to intervene, the zooming locus of the fourthlens unit is made to be allowed to substantially reciprocate. Thus,significant variation of the power can be performed in such a mannerthat a most effective space saving effect can be obtained. Specifically,if the value is larger than the upper limit, the focal length of thefirst lens unit is lengthened, the object point with respect to thesecond lens unit, that is the main unit for varying the power, is maderemote, the imaging magnification of the second lens unit is lowered,and thus effective size reduction cannot be performed. Furthermore, thedistance from the first lens unit to the second lens unit is lengthenedand, therefore, size reduction cannot be performed.

If the value is smaller than the lower limit, the focal length of thefirst lens unit is shortened. If the imaging horizontal magnification ofthe second lens unit is intended to be maintained as described above,the refracting power of the second lens unit is strengthenedexcessively. Thus, a severe accuracy is required to support the lens.

(2-4) It is preferable that the fourth lens unit be moved rearwardstoward the image surface when rangefinding (focusing) of a near objectis performed. If the rangefinding is performed by moving rearwards thefourth lens unit having negative refracting power, the focal length ofthe overall system at the time of performing the foregoing operation issomewhat lengthened. As a result, the problem that the effectivevariable power ratio is reduced in the short distance portion withrespect to the paraxial variable power ratio as has been experiencedwith the rear focusing method in which the positive lens unit performsrangefinding can be prevented. Even if the power is varied considerably,a desired variable power ratio can be obtained in the short distanceportion.

(2-5) It is preferable that the diaphragm relating to the reduction inthe diameter of the front lens be located near the third lens unit atthe telescopic end. At this time, the movement may be performed in sucha manner that the diaphragm is, at the wide angle portion, located at aposition nearer the object than the position at the telescopic end toeffectively reduce the diameter of the front lens.

Generally, light beams out of the axis that are made incident on thefirst lens unit are made highest at an intermediate zooming positionadjacent to the wide angle end because the incident pupil is located ata position deepest (most remote position) from the first lens unit. Ifthe position, at which the light beams are made highest, can be set nearthe wide angle end by moving the incident pupil, that is, the diaphragm,the outer diameter of the first lens unit can be reduced mostefficiently.

In particular, it is preferable that the diaphragm be fixed at aposition near the third lens unit during the zooming to constitute adesired structure. It is preferable that the diaphragm be located in thethird lens unit at a position relatively adjacent to the object, morepreferably located nearest the object in order to reduce the diameter ofthe front lens.

(2-6) Assuming that the distance from the second lens unit to the thirdlens unit (to the surface of the diaphragm in a case where the diaphragmis located in the third lens unit adjacent to the object) at the wideangle end when the distance from the object is infinite is D2_(W), andthe distance from the fourth lens unit to the fifth lens unit is D4_(W),it is preferable that the following be satisfied:

    1.5<D2.sub.W /D4.sub.W <3.5

The foregoing expression provides the space for moving the fourth lensunit for performing focusing, which is enlarged near the telescopic end,in relation to the movable range for the second lens unit, which is themain lens unit for varying the power. In a case of a zoom lens employingthe foregoing rear focusing method, the amount of movement of thefocusing lens at the telescopic end is inevitably enlarged if thedistance from an object is short. In particular, the amount of movementfor correcting the image surface position of the fourth lens unit andthat for performing focusing are enlarged in proportion to the zoomingmagnification. If D2_(W) is large, that is, if the movable range forperforming zooming is large, the zooming ratio is raised. Thus, theamount of movement for the fourth lens unit for performing a desiredclose-up photographing operation must be enlarged at the telescopic end.The foregoing expression indicates the foregoing correlation. If thevalue is larger than the upper limit, the shortest distance is madeexcessively remote at the telescopic end. If the value is smaller thanthe lower limit, an undesired zoom lens having a small zooming ratiowill be realized.

(2-7) It is preferable that the movement of the focusing lens satisfiesthe following:

    0.3<|D4.sub.W /F4|<1.0

The foregoing expression provides an appropriate relationship betweenthe refracting power of the fourth lens unit, which is the focusinglens, and the movable space with respect to the fifth lens unit. If thevalue is larger than the upper limit, an excessively large gap forperforming a desired shortest distance is undesirably provided. Thus,the overall length is lengthened excessively. If the value is smallerthan the lower limit, a space required to perform focusing cannot bemaintained.

In particular, the amount of rearward movement is enlarged at thetelescopic end. To perform power variation exceeding 10 times, it ispreferable that the foregoing ratio be included in the following range:

    0.6<|D4.sub.W /F4|<1.0

It is preferable that the fourth lens unit satisfy the followingrelationship:

    -5.0<F4/F.sub.W <-1.5

If the value is larger than the upper limit, the refracting power of thefourth lens unit is strengthened and the Petzval sum is enlarged in anegative value range. Thus, the image surface is corrected excessively.If the value is smaller than the lower limit, the amount of movement ofthe fourth lens unit is enlarged. As a result, a desired close-upphotography distance cannot be provided. Thus, the overall system isenlarged excessively.

(2-8) It is preferable that the second lens unit, which is the main lensunit for varying the power for performing zooming, satisfies thefollowing:

    -2.0<F2/F.sub.W <-0.8

If the value is larger than the upper limit, the refracting power of thesecond lens unit is strengthened and the Petzval sum is enlarged in anegative value range. Thus, the image surface is corrected excessively.If the value is smaller than the lower limit, the amount of movement ofthe second lens unit for obtaining a desired variable power ratio isenlarged. Thus, the diameter of the front lens and the overall systemare excessively enlarged.

(2-9) It is preferable that light beams be formed into a divergentsystem when they are made incident upon the third lens unit so as to beformed into converged system when they are introduced into the fourthlens unit. That is, it is suitable that the imaging magnification β3always has a negative value over the all zooming region to reduce thesize.

    β3<0

If the foregoing expression is satisfied, the fourth lens unit canefficiently be moved. In particular, the fourth lens unit can be movedto form a locus having convex facing the image surface from the wideangle end to the telescopic end. In particular, the fourth lens unit canbe allowed to substantially completely reciprocate. Thus, the size canbe reduced as desired.

To further reduce the size of the overall system and to effectivelyprovide the moving space, it is preferable that the fourth lens unit beallowed to reciprocate substantially completely or form a locus havingconvex facing the image surface, as described above. To achieve this, itis preferable that the imaging magnification of the third lens unit be anegative value, the absolute value of the same be enlarged from the wideangle end toward the telescopic end, and the absolute value be thenreduced. Assuming that the paraxial horizontal magnifications of thethird lens unit at the wide angle end and that at the telescopic end areβ3w and β3T, the most excellent space efficiency can be obtained in thefollowing case:

    β3W≈β3T

Assuming that the maximum value of the absolute value of the paraxialhorizontal magnification of the third lens unit from the wide angle endto the telescopic end is β3MAX (a value existing during zooming), it ispreferable that the following be satisfied:

    |β3MAX|>0.8

To further enlarge the power, it is preferable that the following besatisfied:

    |β3MAX|>0.9

Furthermore, it is preferable that the foregoing value crosses -1 toenlarge the power.

(2-10) Assuming that the ratio (β2T/β2w) of the magnifications of thesecond lens unit from the wide angle end to the telescopic end is Z2 andthe ratio (Ft/F_(W)) of the focal lengths of the overall system is Z, itis preferable that the following be satisfied:

    0.40<Z2/Z<1.30

The foregoing expression indicates the variable power ratio of thesecond lens unit with respect to the variable power ratio of the overallsystem. If the value is larger than the upper limit, the power of anyunit in the lens system is decreased during zooming. Therefore, thepower variation efficiency deteriorates and the size is undesirablyenlarged. If the value is smaller than the lower limit, a plurality oflens units exist that contribute to the power variation. Thus, each lensunit cannot easily be controlled. If the fourth lens unit is allowed tocompletely reciprocate during zooming, the most excellent space savingefficiency can be performed. To obtain a desired zooming ratio, thefourth lens unit cannot be completely returned sometimes. Also thefourth lens unit contributes to somewhat power variation at theforegoing time (|β4W|<|β4T|). It is preferable that the following besatisfied in the foregoing case:

    0.68<Z2/Z<0.8

If the value is out of the foregoing range and, thus, the fourth lensunit considerably contributes to the power variation, the movable spacefor the fourth lens unit is enlarged. Thus, the size of the overallsystem is enlarged excessively. As a matter of course, if the third lensunit can be moved during zooming and contributes to power variation, theforegoing problems does not arise.

(2-11) It is preferable that the third lens unit satisfy the following:

    1.5<F3/F.sub.W <4.5

The foregoing expression is effective to appropriately provide themagnification of the third lens unit. If the value is larger than theupper limit, the size is enlarged excessively. If the value is smallerthan the lower limit, the movable space for the fourth lens unit forperforming focusing cannot properly be provided. If the value is out ofthe foregoing range, the fourth lens unit cannot be moved efficiently.In particular, the fourth lens unit cannot be moved from the wide angleend to the telescopic end to form a locus having convex facing the imagesurface or cannot be allowed to substantially completely reciprocate.The third lens unit is composed of a single lens to achieve the sizereduction. To prevent spherical aberration, an aspherical surface may beprovided for any surface of the third lens unit.

(2-12) It is preferable that the size and weight of the fourth lensunit, which is a focusing lens unit and which is a lens unit forcorrecting the image point, is moved, be reduced in order to reduce thesize and to simplify the mechanism because the fourth lens unit ismoved. To achieve this, the fourth lens unit may be composed of anegative single lens. To prevent change in the chromatic aberrationoccurring when focusing or zooming is performed, it is preferable thatthe following be satisfied:

    20<ν4<50

where ν4 is the Abbe's number of the material of the lens composing thefourth lens unit.

(2-13) It is preferable that the refractive index of the single lenscomposing the fourth lens unit be included in the following range:

    1.65<N4

where N4 is the refractive index of the lens composing the fourth lensunit. If the value is out of the foregoing range, the curvature ratio ofthe lens surface is sharpened. Thus, the focal length F4 in theforegoing range cannot be provided. Thus, the aberration deterioratesexcessively.

(2-14) To prevent generation of the aberration and to enlarge themovable range, it is preferable that the curvature radius of the lenssurface of the single lens composing the fourth lens unit be included inthe following range:

    1.4<|R4a|/R4b<3.0

(2-15) To cause the fifth lens unit, which is the final imaging lens, tohave an excellent aberration with respect to the focal length of theoverall system and to maintain an appropriate back focus, it ispreferable that the following be satisfied:

    1.50<F5/F.sub.W <5.0

If the value is larger than the upper limit, the back focus islengthened and, thus, the size is enlarged. If the value is smaller thanthe lower limit, the refracting power of the fifth lens unit isstrengthened. Thus, undesirable spherical aberration and comaticaberration will easily be generated in, particularly, in theintermediate zooming region. What is worse, the telecentric relationshipis lost, causing the exit pupil to be shortened undesirably. What isworse, a desired block, such as a filter, cannot be inserted.

(2-16) The structure of the lens according to the present invention hasan arrangement to prevent the distance to the position of the exit pupilof the zoom lens from being excessively short. Furthermore, diagonallight beams from the object are made incident at a small angle asdescribed above so that the diameter of the front lens is reduced.

If the range of the power of the lens unit from the diaphragm toward theimage surface is set to a moderate range, the diagonal light beamscannot be set at a small angle. Thus, the diameter of the front lens isenlarged undesirably. If the value is smaller than the lower limit, theexit pupil is shortened excessively in the positive value range. As aresult, telecentric light beams with respect to the solid-image sensingdevice cannot be maintained. Furthermore, excessive change in theaberration during zooming and focusing takes place. If the value isincluded in the foregoing range, excessive movement can be preventedeven if rangefinding (focusing) is performed. Thus, the size can bereduced. If the value is out of the foregoing range, the diameter of thefront lens is enlarged, causing the size of the overall system to beenlarged. Furthermore, an appropriate emission angle with respect to theimage sensing device cannot be provided. It is preferable that theposition of the exit pupil satisfy the following:

    -1.5<F4/F5<-0.75

If the value is larger than the upper limit, the refracting power of thefourth lens unit is strengthened, and, thus, excessive change in thedistance for performing focusing occurs. If the value is smaller thanthe lower limit, the amount of movement required when focusing isperformed is enlarged. Thus, the size is enlarged excessively. What isworse, the refracting power of the fifth lens unit, which is a fixedimaging lens, is strengthened, causing the exit pupil to be shortenedexcessively.

(2-17) To appropriate maintain the back focus of the lens whileappropriate providing the exit pupil of the overall lens system, it ispreferable that the following be satisfied assuming that the imagingmagnification of the fifth lens unit is β5:

    |β5|<0.25

If the value is out of the foregoing range, the back focus is enlargedand, thus, the lens system is enlarged. What is worse, the exit pupil isshortened undesirably.

(2-18) To cause the fifth lens unit, which is the final imaging lens, tohave an excellent aberration with respect to the focal length of theoverall system and to maintain appropriate back focus, it is preferablethat the following be satisfied:

    0.68<F5/√F.sub.W ×Ft<0.92

If the value is larger than the upper limit, the back focus islengthened, and, therefore, the size is enlarged undesirably. If thevalue is smaller than the lower limit, the refracting power of the fifthlens unit is strengthened, thus causing spherical aberration and comaticaberration to be generated in, particularly, in an intermediate zoomingregion. What is worse, the telecentric relationship is lost, and,therefore, the exit pupil is shortened undesirably.

(2-19) To prevent change in the position of the exit pupil with respectto the solid-image sensing device while maintaining an appropriate backfocus, it is preferable that the magnification β4 of the fourth lensunit always has the same sign during zooming. If β4 is changed with thesign being changed during zooming, the incidental angle on the fixedfifth lens unit, which is the final imaging lens unit, is changedexcessively. Thus, the incidental angle of peripheral light beams on thesolid-image sensing device (a CCD or the like) is changed excessivelywhen the zooming is performed. As a result, excess deviation fromtelecentric imaging takes place, thus causing shading to be generatedundesirably.

(2-20) To prevent shading at the wide angle end and the telescopic endat which the exit pupil is shortened in a range of positive values andto main an appropriate back focus, it is preferable that the followingbe satisfied:

    β4W×β4T>0

where the paraxial horizontal magnifications of the fourth lens unit atthe wide angle end and that at the telescopic end when the distance froman object is infinite are β4W and β4T, respectively.

(2-21) To reduce the size of the overall system, it is preferable thatthe following be satisfied:

    1.5<Bf.sub.W /F.sub.W <4.2

where Bf_(W) is the back focus (element "G", such as a glass block or afilter, in the embodiment is excluded) at the wide angle end when thedistance from an object is infinite. The foregoing expression isrequired to effectively reduce the size of the overall system. If thevalue is smaller than the lower limit, a block, such as a filter, cannotbe inserted. Furthermore, the exit pupil is shortened to undesirablydeviate the image forming on the imaging device from the telecentricsystem. If the value is larger than the upper limit, the size isenlarged undesirably.

(2-22) It is preferable that the specific lens structure of each lensunit be as follows:

The first lens unit is composed of a plurality of lenses including atleast one negative lens.

The second lens unit is composed of a plurality of lenses including atleast one positive lens.

The third lens unit is composed of at least one positive lens.

The fourth lens unit is composed of at least one negative lens.

The fifth lens unit is composed of a plurality of lenses including atleast one negative lens.

In particular, it is preferable that the be composed of a be composed ofa negative meniscus lens having convex facing the object, a positivelens having a convex facing the object and a positive lens having aconvex facing the object, the foregoing lenses being disposed in theforegoing sequential order when viewed from the object.

It is preferable that the second lens unit be composed of a negativemeniscus lens having a convex facing the object, a double-concave lensand a positive lens, the foregoing lenses being disposed in theforegoing sequential order when viewed from the object.

An aspherical surface may be provided for any lens surface of the thirdlens unit. If the third lens unit is composed of a single lens, theaspherical surface may be provided for its surface facing the object orfor its surface facing the image surface. To satisfactorily correct thespherical aberration, it is preferable that the aspherical surface beprovided on the lens surface facing the object,

The fourth lens unit corrects change in the image point of the secondlens unit occurring during zooming, or that occurring when the secondlens unit is moved, and as well as performs focusing. To prevent changein the aberration occurring during focusing, and to prevent change inthe chromatic aberration, the fourth lens unit may be composed of twolenses consisting of a positive lens and a negative lens, in place ofbeing composed of a single lens.

The positive lens and the negative lens may be bonded so as to be formedinto one block. In this case, it is preferable that the following besatisfied assuming that the Abbe's number of the material of thepositive lens and that of the negative lens are ν4p and ν4n,respectively:

    ν4p<ν4n

If the value is out of the foregoing range, the chromatic aberration andthe distance are changed excessively. By composing the fourth lens unitby bonding a positive lens and a negative lens to each other, the fourthlens unit, which is moved when zooming is performed and also moved whenfocusing is performed, is formed into a single block. Thus, thestructure of the fourth lens unit can be simplified and supported easilysimilarly to the case where the fourth lens unit is composed of a singlelens.

Assuming that the Abbe's numbers of the positive lens and the negativelens composing the fourth lens unit are ν4p and ν4n, it is preferablethat the following be satisfied:

    ν4n-ν4p>15

To effectively change the chromatic aberration during zooming, it ispreferable that the following be satisfied:

    ν4n-ν4p>30

In a case where the lenses for composing the fourth lens unit areunified into one block, in particular, in a case where the same iscomposed by bonding a positive lens and a negative lens or composed ofone negative lens, it is preferable that the lens surface of the blockfacing an object has the concave facing the object and the lens surfacefacing the image has the concave facing the image.

The fifth lens unit is the positive lens unit that is fixed duringzooming. Thus, spherical aberration can easily be generated. To correctthe spherical aberration, it is preferable that the fifth lens unit becomposed of at least one positive lens and at least one negative lens.If the fifth lens unit is composed of one positive lens and one negativelens, it is preferable that an aspherical surface be provided for eitherlens surface. In this case, the spherical aberration can satisfactorilybe corrected. To correct the chromatic aberration, a bonded lens unitmay be employed.

If the fifth lens unit is composed of one positive lens and one negativelens, it is preferable that the structure be employed in which anegative meniscus lens having a convex surface facing the object and adouble-convex lens are disposed in this sequential order when viewedfrom the object. As a result, the second principal point is located in aportion adjacent to the image surface. Thus, the back focus caneffectively be maintained. In this case, the two lenses may be bonded toeach other so as to be formed into one block. Thus, the lens unit caneasily be supported and thus the structure can be simplified.

(2-23) To satisfactorily shorten the distance e1 between the principalpoints of the first lens unit and that of the second lens unit for thepurpose of widen the angle is one of important points. To achieve this,it is preferable that the second lens unit has the following structure.

To shorten e1 at the wide angle end, it is preferable that the (second)principal point of the first lens unit facing the image surface belocated at a position facing the image surface for the purpose of widenthe angle. Specifically, a negative meniscus lens L11 having a convexsurface facing an object, an air gap, a positive lens L12 having aconvex surface facing the object, and a positive lens L13 having aconvex surface facing the object are disposed in the foregoingsequential order when viewed from a position adjacent to the first lensunit facing the object. It is preferable that an air lens formed by thelenses L11 and L12 has negative refracting power. Thus, the principalpoint of the first lens unit facing the image surface is shifted towardthe second lens unit. As a result, e1 can be shortened at the wide angleportion so that angle is widened effectively. To further shorten e1 atthe wide angle end, it is preferable that the (first) principal point ofthe second lens unit-facing the object be located in a portion adjacentto the object in order to widen the angle.

Specifically, it is preferable that a negative meniscus lens L21 havinga convex surface facing an object, a double-concave negative lens L21,an air gap, and a positive lens L23 be disposed in the foregoingsequential order when viewed from a portion of the second lens unitfacing the object. The air gap shifts the principal point of the secondlens unit facing the object toward the first lens unit. Thus, e1 can beshortened in the wide angle portion so that angle is widenedeffectively. A negative lens L24 may be disposed in a portion adjacentto the lens L23 facing the image surface. The lens L24 shifts theprincipal point of the second lens unit facing the object toward thefirst lens unit. Furthermore, change in the chromatic aberrationoccurring during zooming can be prevented effectively.

(2-24) To realize a small size and wide angle zoom lens, it ispreferable that the following be satisfied assuming that the outerdiameter of the lens nearest the object is φ1 and the focal length ofthe overall system at the wide angle end is F_(W) :

    0.10<F.sub.W /φ1<0.20

Generally, if the focal length F_(W) at the wide angle end is shortened,the diameter φ1 of the front lens is enlarged. If the focal length F_(W)at the wide angle end is somewhat lengthened, the diameter φ1 of thefront lens is reduced though the reduction depends upon the brightnessof the lens system at the telescopic end. The foregoing expressionindicates an appropriate relationship between the diameter of the frontlens and the focal length at the wide angle end for the purpose ofrealizing a small size and wide angle zoom lens. If the value is out ofthe foregoing range, a small size and wide angle zoom lens cannot beprovided. Specifically, if the value is larger than the upper limit, azoom lens of a telescopic type is realized. If the value is smaller thanthe lower limit, a large size zoom lens will be realized undesirably.

The diameter φ1 of the front lens is like the optically effectivediameter of the first lens unit and is an actual outer diameter of thelens. The diameter φ1 of the front lens is larger than the opticallyeffective diameter of the first lens unit by about 0% to 6%.

According to the present invention, at least any of the foregoingconditions (2-1) to (2-24) is satisfied so that the diameter of thefront lens is reduced. Furthermore, a rear focusing zoom lens can beprovided which exhibits: the diameter of the front lens can be reduced;the angle can be widened at variable power ratios of about 6 times toabout 20 times; the structure including the mechanism can be simplified;the size and weight can be reduced; the actual variable power ratiocannot easily be changed when the distance from an object is aneffective distance (short distance); excellent performance can beobtained over all zooming regions over the distance from the object, inparticular, distortion can be prevented; the size can be reduced and theangle can be widened; the lens structure can be simplified; and themanufacturing cost can be reduced.

Examples of the present invention will now be described. In theexamples, Ri is the curvature radius of the lens system of the i-th lenscounted from the object, Di is the thickness of the i-th lens and thatof an air gap, and Ni and νi are the refractive index of the glass ofthe i-th lens counted from the object and the Abbe's number of the same.In the examples, the final two lens surfaces are glass blocks, such as aface plate and a filter. The relationship among the foregoingconditional expressions and the specifications of the examples are shownin Table 3. Under conditions that X-axis stands for the direction of theoptical axis, H-axis stands for the direction perpendicular to theoptical axis, the direction of travel of light is expressed by positivevalues, R represents the paraxial curvature radius, and K, B, C, D, andE are aspherical surface coefficients, the aspherical surface isexpressed by the following equation: ##EQU3##

EXAMPLE

    __________________________________________________________________________    f = 1˜5.98        Fno = 1.85˜2.17          2ω =             61.7°˜11.4°                                               __________________________________________________________________________    R1  = 5.858 D1  = 0.28 N1  = 1.846659                                                                         ν1  = 23.8                                 R2  = 3.628 D2  = 1.26 N2  = 1.603112                                                                         ν2  = 60.7                                 R3  = -478.413                                                                            D3  = 0.04                                                        R4  = 2.985 D4  = 0.73 N3  = 1.603112                                                                         ν3  = 60.7                                 R5  = 7.503 D5  = VARIABLE                                                    R6  = 7.219 D6  = 0.12 N4  = 1.806098                                                                         ν4  = 41.0                                 R7  = 0.994 D7  = 0.44                                                        R8  = -2.897                                                                              D8  = 0.12 N5  = 1.806098                                                                         ν5  = 41.0                                 R9  = 2.028 D9  = 0.21                                                        R10 = 2.279 D10 = 0.44 N6  = 1.846659                                                                         ν6  = 23.8                                 R11 = 36.548                                                                              D11 = VARIABLE                                                    R12 = DIAPHRAGM                                                                           D12 = 0.27                                                        R13 = 2.069 D13 = 0.73 N7  = 1.583126                                                                         ν7  = 59.4                                 R14 = -1.988                                                                              D14 = VARIABLE                                                    R15 = -4.254                                                                              D15 = 0.20 N8  = 1.806098                                                                         ν8  = 41.0                                 R16 = 2.642 D16 = VARIABLE                                                    R17 = 3.390 D17 = 0.49 N9  = 1.516330                                                                         ν9  = 64.2                                 R18 = -2.961                                                                              D18 = 0.04                                                        R19 = 2.046 D19 = 0.15 N10 = 1.846659                                                                         ν10 = 23.8                                 R20 = 1.158 D20 = 0.61 N11 = 1.487490                                                                         ν11 = 70.2                                 R21 = -6.436                                                                              D21 = 0.49                                                        R22 = ∞                                                                             D22 = 1.05 N12 = 1.516330                                                                         ν12 = 64.2                                 R23 = ∞                                                                 __________________________________________________________________________     G = R22, R23                                                             

    FOCAL LENGTH                                                                  VARIABLE DISTANCE                                                                        1.00        3.84                                                                             5.98                                                __________________________________________________________________________    D5         0.20        1.79                                                                             2.24                                                D11        2.35        0.76                                                                             0.30                                                D14        0.29        0.78                                                                             0.61                                                D16        0.86        0.37                                                                             0.54                                                __________________________________________________________________________    ASPHERICAL COEFFICIENT                                                        __________________________________________________________________________    R13   K = -1.659e + 00                                                                         B = -4.121e - 02                                                                         C = -1.083e - 02                                        D =   1.125e - 02                                                                        E = -1.238e - 02                                             __________________________________________________________________________

EXAMPLE

    __________________________________________________________________________    f = 1˜6.00        Fno = 1.85˜2.08          2ω =             61.7°˜11.4°                                               __________________________________________________________________________    R1  = 5.850 D1  = 0.28 N1  = 1.846659                                                                         ν1  = 23.8                                 R2  = 3.656 D2  = 1.36 N2  = 1.603112                                                                         ν2  = 60.7                                 R3  = -165.992                                                                            D3  = 0.04                                                        R4  = 3.005 D4  = 0.74 N3  = 1.603112                                                                         ν3  = 60.7                                 R5  = 7.300 D5  = VARIABLE                                                    R6  = 7.031 D6  = 0.12 N4  = 1.806098                                                                         ν4  = 41.0                                 R7  = 0.994 D7  = 0.46                                                        R8  = -2.861                                                                              D8  = 0.12 N5  = 1.806098                                                                         ν5  = 41.0                                 R9  = 2.026 D9  = 0.21                                                        R10 = 2.259 D10 = 0.44 N6  = 1.846659                                                                         ν6  = 23.8                                 R11 = 38.408                                                                              D11 = VARIABLE                                                    R12 = DIAPHRAGM                                                                           D12 = 0.27                                                        R13 = 2.089 D13 = 0.73 N7  = 1.583126                                                                         ν7  = 59.4                                 R14 = -1.995                                                                              D14 = VARIABLE                                                    R15 = -4.166                                                                              D15 = 0.20 N8  = 1.806098                                                                         ν8  = 41.0                                 R16 = 2.611 D16 = VARIABLE                                                    R17 = 3.368 D17 = 0.49 N9  = 1.516330                                                                         ν9  = 64.2                                 R18 = -2.981                                                                              D18 = 0.04                                                        R19 = 2.034 D19 = 0.15 N10 = 1.846659                                                                         ν10 = 23.8                                 R20 = 1.160 D20 = 0.61 N11 = 1.487490                                                                         ν11 = 70.2                                 R21 = -5.416                                                                              D21 = 0.49                                                        R22 = ∞                                                                             D22 = 1.05 N12 = 1.516330                                                                         ν12 = 64.2                                 R23 = ∞                                                                 __________________________________________________________________________     G = R22, R23                                                             

    FOCAL LENGTH                                                                  VARIABLE DISTANCE                                                                        1.00        3.87                                                                             6.00                                                __________________________________________________________________________    D5         0.20        1.79                                                                             2.24                                                D11        2.35        0.76                                                                             0.30                                                D14        0.33        0.80                                                                             0.59                                                D16        0.84        0.37                                                                             0.57                                                __________________________________________________________________________    ASPHERICAL COEFFICIENT                                                        __________________________________________________________________________    R13   K = -1.659e + 00                                                                         B = -4.141e - 02                                                                         C = -1.082e - 02                                        D =   1.125e - 02                                                                        E = -1.238e - 02                                             __________________________________________________________________________

EXAMPLE

    __________________________________________________________________________    f = 1˜6.00        Fno = 1.85˜2.04          2ω =             61.7°˜11.4°                                               __________________________________________________________________________    R1  = 6.215 D1  = 0.28 N1  = 1.846659                                                                         ν1  = 23.8                                 R2  = 3.900 D2  = 1.42 N2  = 1.603112                                                                         ν2  = 60.7                                 R3  = -111.124                                                                            D3  = 0.04                                                        R4  = 2.879 D4  = 0.84 N3  = 1.603112                                                                         ν3  = 60.7                                 R5  = 6.193 D5  = VARIABLE                                                    R6  = 5.998 D6  = 0.12 N4  = 1.806098                                                                         ν4  = 41.0                                 R7  = 0.991 D7  = 0.46                                                        R8  = -2.633                                                                              D8  = 0.12 N5  = 1.806098                                                                         ν5  = 41.0                                 R9  = 2.041 D9  = 0.21                                                        R10 = 2.343 D10 = 0.44 N6  = 1.846659                                                                         ν6  = 23.8                                 R11 = 42.205                                                                              D11 = VARIABLE                                                    R12 = DIAPHRAGM                                                                           D12 = 0.27                                                        R13 = 2.270 D13 = 0.73 N7  = 1.583126                                                                         ν7  = 59.4                                 R14 = -1.862                                                                              D14 = VARIABLE                                                    R15 = -6.532                                                                              D15 = 0.20 N8  = 1.806098                                                                         ν8  = 41.0                                 R16 = 2.829 D16 = VARIABLE                                                    R17 = 1.873 D17 = 0.15 N9  = 1.846659                                                                         ν9  = 23.8                                 R18 = 1.085 D18 = 0.85 N10 = 1.583126                                                                         ν10 = 59.4                                 R19 = -2.044                                                                              D19 = 0.49                                                        R20 = ∞                                                                             D20 = 1.05 N11 = 1.516330                                                                         ν11 = 64.2                                 R21 = ∞                                                                 __________________________________________________________________________     G = R20, R21                                                             

    FOCAL LENGTH                                                                  VARIABLE DISTANCE                                                                        1.00        3.76                                                                             6.00                                                __________________________________________________________________________    D5         0.20        1.79                                                                             2.24                                                D11        2.35        0.76                                                                             0.30                                                D14        0.31        0.81                                                                             0.63                                                D16        1.14        0.64                                                                             0.82                                                __________________________________________________________________________    ASPHERICAL COEFFICIENT                                                        __________________________________________________________________________    R13   K = -1.654e + 00                                                                         B = -5.235e - 02                                                                         C = -1.093e - 02                                        D =   1.205e - 02                                                                        E = -1.465e - 02                                             R19   K = -5.996e - 01                                                                         B =   2.686e - 02                                                                        C = -6.032e - 03                                        D = -1.379e - 02                                                                         E = -1.860e - 03                                             __________________________________________________________________________

EXAMPLE

    __________________________________________________________________________    f = 1˜5.98        Fno = 1.85˜2.10          2ω =             61.7°˜11.4°                                               __________________________________________________________________________    R1  = 6.182 D1  = 0.28 N1  = 1.846659                                                                         ν1  = 23.8                                 R2  = 4.042 D2  = 1.72 N2  = 1.603112                                                                         ν2  = 60.7                                 R3  = -391.245                                                                            D3  = 0.04                                                        R4  = 2.986 D4  = 0.98 N3  = 1.603112                                                                         ν3  = 60.7                                 R5  = 6.235 D5  = VARIABLE                                                    R6  = 6.112 D6  = 0.12 N4  = 1.806098                                                                         ν4  = 41.0                                 R7  = 0.975 D7  = 0.46                                                        R8  = -2.510                                                                              D8  = 0.12 N5  = 1.806098                                                                         ν5  = 41.0                                 R9  = 2.066 D9  = 0.21                                                        R10 = 2.259 D10 = 0.44 N6  = 1.846659                                                                         ν6  = 23.8                                 R11 = 26.760                                                                              D11 = VARIABLE                                                    R12 = DIAPHRAGM                                                                           D12 = 0.27                                                        R13 = 2.257 D13 = 0.73 N7  = 1.583126                                                                         ν7  = 59.4                                 R14 = -1.874                                                                              D14 = VARIABLE                                                    R15 = -6.651                                                                              D15 = 0.20 N8  = 1.806098                                                                         ν8  = 41.0                                 R16 = 2.890 D16 = VARIABLE                                                    R17 = 1.825 D17 = 0.15 N9  = 1.846659                                                                         ν9  = 23.8                                 R18 = 1.057 D18 = 0.85 N10 = 1.583126                                                                         ν10 = 59.4                                 R19 = -1.989                                                                              D19 = 0.49                                                        R20 = ∞                                                                             D20 = 1.05 N11 = 1.516330                                                                         ν11 = 64.2                                 R21 = ∞                                                                 __________________________________________________________________________     G = R20, R21                                                             

    FOCAL LENGTH                                                                  VARIABLE DISTANCE                                                                        1.00        3.74                                                                             5.98                                                __________________________________________________________________________    D5         0.20        1.76                                                                             2.20                                                D11        2.31        0.75                                                                             0.30                                                D14        0.30        0.86                                                                             0.72                                                D16        1.02        0.46                                                                             0.60                                                __________________________________________________________________________    ASPHERICAL COEFFICIENT                                                        __________________________________________________________________________    R13   K = -1.654e + 00                                                                         B = -5.302e - 02                                                                         C = -1.100e - 02                                        D =   1.205e - 02                                                                        E = -1.465e - 02                                             R19   K = -5.996e - 01                                                                         B =   2.852e - 02                                                                        C = -5.979e - 03                                        D = -1.379e - 02                                                                         D = -1.860e - 03                                             __________________________________________________________________________

EXAMPLE

    __________________________________________________________________________    f = 1˜5.97        Fno = 1.85˜2.13          2ω =             61.7°˜11.4°                                               __________________________________________________________________________    R1  = 6.454 D1  = 0.28 N1  = 1.846659                                                                         ν1  = 23.8                                 R2  = 4.213 D2  = 2.16 N2  = 1.603112                                                                         ν2  = 60.7                                 R3  = 273.515                                                                             D3  = 0.04                                                        R4  = 2.979 D4  = 1.01 N3  = 1.603112                                                                         ν3  = 60.7                                 R5  = 6.298 D5  = VARIABLE                                                    R6  = 5.324 D6  = 0.12 N4  = 1.772499                                                                         ν4  = 49.6                                 R7  = 0.927 D7  = 0.46                                                        R8  = -2.452                                                                              D8  = 0.12 N5  = 1.806098                                                                         ν5  = 41.0                                 R9  = 2.041 D9  = 0.21                                                        R10 = 2.255 D10 = 0.44 N6  = 1.846659                                                                         ν6  = 23.8                                 R11 = 33.368                                                                              D11 = VARIABLE                                                    R12 = DIAPHRAGM                                                                           D12 = 0.27                                                        R13 = 2.269 D13 = 0.73 N7  = 1.583126                                                                         ν7  = 59.4                                 R14 = -1.858                                                                              D14 = VARIABLE                                                    R15 = -7.346                                                                              D15 = 0.20 N8  = 1.846659                                                                         ν8  = 23.8                                 R16 = 3.047 D16 = VARIABLE                                                    R17 = 1.894 D17 = 0.15 N9  = 1.846659                                                                         ν9  = 23.8                                 R18 = 1.273 D18 = 0.02                                                        R19 = 1.353 D19 = 0.85 N10 = 1.583126                                                                         ν10 = 59.4                                 R20 = -1.990                                                                              D20 = 0.49                                                        R21 = ∞                                                                             D21 = 1.05 N11 = 1.516330                                                                         ν11 = 64.2                                 R22 = ∞                                                                 __________________________________________________________________________     G = R21, R22                                                             

    FOCAL LENGTH                                                                  VARIABLE DISTANCE                                                                        1.00        3.71                                                                             5.97                                                __________________________________________________________________________    D5         0.20        1.79                                                                             2.23                                                D11        2.34        0.75                                                                             0.30                                                D14        0.29        0.86                                                                             0.76                                                D16        1.06        0.49                                                                             0.59                                                __________________________________________________________________________    ASPHERICAL COEFFICIENT                                                        __________________________________________________________________________    R13   K = -1.654e + 00                                                                         B = -5.606e - 02                                                                         C = -1.108e - 02                                        D =   1.205e - 02                                                                        E = -1.465e - 02                                             R20   K = -5.997e - 01                                                                         B =   3.118e - 02                                                                        C = -5.914e - 03                                        D = -1.378e - 02                                                                         D = -1.860e - 03                                             __________________________________________________________________________

EXAMPLE

    __________________________________________________________________________    f = 1˜5.97        Fno = 1.85˜2.15          2ω =             61.7°˜11.4°                                               __________________________________________________________________________    R1  = 6.447 D1  = 0.28 N1  = 1.846659                                                                         ν1  = 23.8                                 R2  = 4.213 D2  = 2.16 N2  = 1.603112                                                                         ν2  = 60.7                                 R3  = 283.391                                                                             D3  = 0.04                                                        R4  = 3.153 D4  = 1.01 N3  = 1.638539                                                                         ν3  = 55.4                                 R5  = 6.691 D5  = VARIABLE                                                    R6  = 5.555 D6  = 0.12 N4  = 1.806098                                                                         ν4  = 41.0                                 R7  = 0.968 D7  = 0.46                                                        R8  = -2.452                                                                              D8  = 0.12 N5  = 1.806098                                                                         ν5  = 41.0                                 R9  = 2.041 D9  = 0.21                                                        R10 = 2.255 D10 = 0.44 N6  = 1.846659                                                                         ν6  = 23.8                                 R11 = 33.376                                                                              D11 = VARIABLE                                                    R12 = DIAPHRAGM                                                                           D12 = 0.27                                                        R13 = 2.269 D13 = 0.73 N7  = 1.583126                                                                         ν7  = 59.4                                 R14 = -1.858                                                                              D14 = VARIABLE                                                    R15 = -7.346                                                                              D15 = 0.20 N8  = 1.846659                                                                         ν8  = 23.8                                 R16 = 3.046 D16 = VARIABLE                                                    R17 = 1.894 D17 = 0.15 N9  = 1.846659                                                                         ν9  = 23.8                                 R18 = 1.273 D18 = 0.02                                                        R19 = 1.353 D19 = 0.85 N10 = 1.583126                                                                         ν10 = 59.4                                 R20 = -2.013                                                                              D20 = 0.49                                                        D21 = ∞                                                                             D21 = 1.05 N11 = 1.516330                                                                         ν11 = 64.2                                 R22 = ∞                                                                 __________________________________________________________________________     G = R21, R22                                                             

    FOCAL LENGTH                                                                  VARIABLE DISTANCE                                                                        1.00        3.71                                                                             5.97                                                __________________________________________________________________________    D5         0.20        1.79                                                                             2.24                                                D11        2.35        0.75                                                                             0.30                                                D14        0.29        0.87                                                                             0.78                                                D16        1.06        0.48                                                                             0.57                                                __________________________________________________________________________    ASPHERICAL COEFFICIENT                                                        __________________________________________________________________________    R13   K = -1.654e + 00                                                                         B = -5.606e - 02                                                                         C = -1.108e - 02                                        D =   1.205e - 02                                                                        E = -1.465e - 02                                             R20   K = -5.997e - 01                                                                         B =   3.118e - 02                                                                        C = -5.914e - 03                                        D = -1.378e - 02                                                                         D = -1.860e - 03                                             __________________________________________________________________________

EXAMPLE

    __________________________________________________________________________    f = 1˜20.05        Fno = 1.85˜3.59          2ω =            62.9°˜3.5°                                                __________________________________________________________________________    R1  = 15.447                                                                              D1  = 0.30 N1  = 1.846659                                                                         ν1  = 23.8                                 R2  = 7.375 D2  = 0.23 N2  = 1.603112                                                                         ν2  = 60.7                                 R3  = 10.645                                                                              D3  = 0.84                                                        R4  = -36.500                                                                             D4  = 0.05 N3  = 1.603112                                                                         ν3  = 60.7                                 R5  = 5.434 D5  = 1.05                                                        R6  = 53.752                                                                              D6  = VARIABLE                                                    R7  = 36.191                                                                              D7  = 0.15 N4  = 1.882997                                                                         ν4  = 40.8                                 R8  = 1.718 D8  = 0.50                                                        R9  = -5.552                                                                              D9  = 0.15 N5  = 1.834000                                                                         ν5  = 37.2                                 R10 = 3.669 D10 = 0.27                                                        R11 = 3.494 D11 = 0.62 N6  = 1.846659                                                                         ν6  = 23.8                                 R12 = -3.680                                                                              D12 = 0.04                                                        R13 = -3.363                                                                              D13 = 0.17 N7  = 1.785896                                                                         ν7  = 44.2                                 R14 = 15.751                                                                              D14 = VARIABLE                                                    R15 = DIAPHRAGM                                                                           D15 = VARIABLE                                                    R16 = 14.991                                                                              D16 = 0.68 N8  = 1.583126                                                                         ν8  = 59.4                                 R17 = -4.832                                                                              D17 = 0.05                                                        R18 = 7.603 D18 = 0.93 N9  = 1.719995                                                                         ν9  = 50.3                                 R19 = -2.730                                                                              D19 = 0.25 N10 = 1.846659                                                                         ν10 = 23.8                                 R20 = -10.068                                                                             D20 =                                                             R21 = -4.381                                                                              D21 = 0.11 N11 = 1.603112                                                                         ν11 = 60.7                                 R22 = 2.530 D22 = 0.50 N12 = 1.846659                                                                         ν12 = 23.8                                 R23 = 3.871 D23 =                                                             R24 = 12.568                                                                              D24 = 0.82 N13 = 1.583126                                                                         ν13 = 59.4                                 R25 = -4.243                                                                              D25 = 0.05                                                        R26 = 3.627 D26 = 0.17 N14 = 1.846659                                                                         ν14 = 23.8                                 R27 = 2.027 D27 = 0.12                                                        R28 = 2.151 D28 = 0.87 N15 = 1.516330                                                                         ν15 = 64.2                                 R29 = -32.087                                                                             D29 = 0.50                                                        R30 = ∞                                                                             D30 = 1.08 N16 = 1.516330                                                                         ν16 = 64.2                                 R31 = ∞                                                                 __________________________________________________________________________     G = R30, R31                                                             

    FOCAL LENGTH                                                                  VARIABLE DISTANCE                                                                        1.00        8.45                                                                             20.05                                               __________________________________________________________________________    D6         0.22        4.59                                                                             5.82                                                D14        5.95        1.58                                                                             0.35                                                D15        1.76        0.79                                                                             0.52                                                D20        0.54        2.93                                                                             1.74                                                D23        2.54        1.12                                                                             2.59                                                __________________________________________________________________________

EXAMPLE

    __________________________________________________________________________    f = 1˜19.70        Fno = 1.65˜2.84          2ω =            62.1°˜3.5°                                                __________________________________________________________________________    R1  = 15.257                                                                              D1  = 0.30 N1  = 1.846659                                                                         ν1  = 23.8                                 R2 = 7.634  D2  = 0.36                                                        R3 = 10.870 D3  = 0.99 N2  = 1.603112                                                                         ν2  = 60.7                                 R4 = -84.464                                                                              D4  = 0.05                                                        R5 = 6.156  D5  = 1.33 N3  = 1.603112                                                                         ν3  = 60.7                                 R6 = 95.517 D6  = VARIABLE                                                    R7 = 22.162 D7  = 0.15 N4  = 1.882997                                                                         ν4  = 40.8                                 R8 = 1.597  D8  = 0.50                                                        R9 = -4.863 D9  = 0.15 N5  = 1.834000                                                                         ν5  = 37.2                                 R10 = 4.907 D10 = 0.27                                                        R11 = 3.799 D11 = 0.62 N6  = 1.846659                                                                         ν6  = 23.8                                 R12 = -4.675                                                                              D12 = 0.03                                                        R13 = -3.804                                                                              D13 = 0.17 N7  = 1.785896                                                                         ν7  = 44.2                                 R14 = -217.759                                                                            D14 = VARIABLE                                                    R15 = DIAPHRAGM                                                                           D15 = 0.42                                                        R16 = 53.443                                                                              D16 = 0.85 N8  = 1.583126                                                                         ν8  = 59.4                                 R17 = -4.258                                                                              D17 = 0.05                                                        R18 = 5.595 D18 = 1.05 N9  = 1.719995                                                                         ν9  = 50.3                                 R19 = -2.961                                                                              D19 = 0.25 N10 = 1.846659                                                                         ν10 = 23.8                                 R20 = -12.916                                                                             D20 = VARIABLE                                                    R21 = -4.385                                                                              D21 = 0.17 N11 = 1.603112                                                                         ν11 = 60.7                                 R22 = 1.992 D22 = 0.49 N12 = 1.846659                                                                         ν12 = 23.8                                 R23 = -3.225                                                                              D23 = VARIABLE                                                    R24 = -9.664                                                                              D24 = 0.75 N13 = 1.583126                                                                         ν13 = 59.4                                 R25 = -4.531                                                                              D25 = 0.05                                                        R26 = 3.530 D26 = 0.17 N14 = 1.846659                                                                         ν14 = 23.8                                 R27 = 1.790 D27 = 0.03                                                        R28 = 1.812 D28 = 1.06 N15 = 1.516330                                                                         ν15 = 64.2                                 R29 = -18.764                                                                             D29 = 0.49                                                        R30 = ∞                                                                             D30 = 1.07 N16 = 1.516330                                                                         ν16 = 64.2                                 R31 = ∞                                                                 __________________________________________________________________________     G = R30, R31                                                             

    FOCAL LENGTH                                                                  VARIABLE DISTANCE                                                                        1.00        7.66                                                                             19.70                                               __________________________________________________________________________    D6         0.22        5.40                                                                             6.86                                                D14        7.28        2.11                                                                             0.65                                                D20        0.50        2.22                                                                             1.12                                                D23        2.68        0.96                                                                             2.06                                                __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________    CONDITIONAL                                                                            EXAMPLES                                                             EXPRESSIONS                                                                            20  21 22 23 24 25 26   27                                           __________________________________________________________________________    |R4a|/R4b                                                            1.61                                                                              1.60                                                                             2.31                                                                             2.30                                                                             2.41                                                                             2.41                                                                             1.13 1.36                                         F1/Fw    4.98                                                                              5.00                                                                             5.11                                                                             5.30                                                                             5.52                                                                             5.51                                                                             8.64 9.81                                         D2w/D4w  2.74                                                                              2.80                                                                             2.07                                                                             2.27                                                                             2.21                                                                             2.21                                                                             2.34 2.72                                         |D4w/F4|                                                             0.43                                                                              0.43                                                                             0.47                                                                             0.41                                                                             0.42                                                                             0.42                                                                             0.67 0.75                                         F4/Fw    -2.00                                                                             -1.97                                                                            -2.43                                                                            -2.48                                                                            -2.52                                                                            -2.52                                                                            -3.78                                                                              -3.56                                        F2/Fw    -0.99                                                                             -0.99                                                                            -0.97                                                                            -0.94                                                                            -0.95                                                                            -0.95                                                                            -1.45                                                                              -1.53                                        β3                                                                           β3W                                                                           -0.57                                                                             -0.57                                                                            -0.58                                                                            -0.59                                                                            -0.59                                                                            -0.58                                                                            -0.47                                                                              -0.43                                            β3T                                                                           -0.73                                                                             -0.71                                                                            -0.75                                                                            -0.81                                                                            -0.84                                                                            -0.84                                                                            -0.81                                                                              -0.61                                        Z2/Z     0.75                                                                              0.79                                                                             0.78                                                                             0.73                                                                             0.70                                                                             0.70                                                                             0.58 0.71                                         F3/Fw    1.86                                                                              1.87                                                                             1.88                                                                             1.88                                                                             1.87                                                                             1.87                                                                             3.58 3.44                                         ν4    41.0                                                                              41.0                                                                             41.0                                                                             41.0                                                                             23.8                                                                             23.8                                                                             --   --                                           N4       1.81                                                                              1.81                                                                             1.81                                                                             1.81                                                                             1.85                                                                             1.85                                                                             --   --                                           F5/Fw    2.06                                                                              2.00                                                                             2.18                                                                             2.13                                                                             2.12                                                                             2.13                                                                             3.95 3.80                                         F4/F5    -0.97                                                                             -0.98                                                                            -1.11                                                                            -1.17                                                                            -1.19                                                                            -1.18                                                                            -0.96                                                                              -0.94                                        |β5|                                                            0.15                                                                              0.16                                                                             0.03                                                                             0.03                                                                             0.05                                                                             0.05                                                                             0.001                                                                              0.05                                          ##STR4##                                                                              0.84                                                                              0.82                                                                             0.89                                                                             0.87                                                                             0.87                                                                             0.87                                                                             0.88 0.86                                         β4                                                                           β4W                                                                           -6.3                                                                              -5.9                                                                             33.1                                                                             31.0                                                                             19.0                                                                             18.2                                                                             1444 20.6                                             β4T                                                                           -6.4                                                                              -6.0                                                                             33.0                                                                             31.0                                                                             18.8                                                                             18.0                                                                             1444 20.6                                         FW/φ1                                                                              0.18                                                                              0.18                                                                             0.17                                                                             0.15                                                                             0.13                                                                             0.13                                                                             0.17 0.14                                         __________________________________________________________________________

As described above, according to the present invention, there isprovided a rear focusing zoom lens that is characterized in that: therear focusing method is employed, a large aperture ratio is realizedsuch that the F-number at the wide angle end is about 1.8; a wide angleis realized such that the photographing image angle at the wide angleend is 60 degrees or more; a significant power variation is realizedsuch that the variable power ratio is 6 times to 20 times; the diameterof the front lens can be reduced; enlargement of the size of the overallsystem can be prevented; excellent optical performance are realized overthe power variation range from the wide angle end to the telescopic endand over the distance from the object in a range from infinite distanceto a near distance, and in particular, distortion can be prevented; andthe structure can be simplified.

Although the invention has been described in its preferred-form with acertain degree of particularity, it is understood that the presentdisclosure of the preferred form can be changed in the details ofconstruction and the combination and arrangement of parts may beresorted to without departing from the spirit and the scope of theinvention as hereinafter claimed.

What is claimed is:
 1. A zoom lens consisting of in the following orderfrom the object side:a first lens unit which is fixed in position duringzooming and has a positive refractive power; a second lens unit which ismovable upon zooming and has a negative refractive power; a third lensunit which is fixed in position during zooming and has a positiverefractive power; a fourth lens unit which is movable upon zooming andhas a negative refractive power; and a fifth lens unit which is fixed inposition during zooming and has a positive refractive power, wherein,upon zooming from the wide-angle end to the telephoto end, said secondlens unit is unidirectionally moved from the object side to the imageside along an optical axis, and said fourth lens unit is moved along theoptical axis from the object side to the image side within a range fromthe wide-angle end to a middle position between the wide-angle andtelephoto ends, and is moved along the optical axis from the image sideto the object side within a range from the middle position to thetelephoto end.
 2. A zoom lens comprising in the following order from theobject side:a first lens unit which is fixed in position during zoomingand has a positive refractive power; a second lens unit which is movableupon zooming and has a negative refractive power; a third lens unitwhich is fixed in position during zooming and has a positive refractivepower; a fourth lens unit which is movable upon zooming and has anegative refractive power; and a fifth lens unit which is fixed inposition during zooming and has a positive refractive power, wherein,upon zooming from the wide-angle end to the telephoto end, said secondlens unit is unidirectionally moved from the object side to the imageside along an optical axis, and said fourth lens unit is moved along theoptical axis from the object side to the image side within a range fromthe wide-angle end to a middle position between the wide-angle andtelephoto ends, and is moved along the optical axis from the image sideto the object side within a range from the middle position to thetelephoto end, wherein said zoom lens satisfies one of the following:

    (F.sub.T).sup.1/2 ·f.sub.1 /f.sub.T =0.833 and |β.sub.2W ·V.sup.1/2 |=0.951;

    (F.sub.T).sup.1/2 ·f.sub.1 /f.sub.T =0.880 and |β.sub.2W ·V.sup.1/2 |=1.044;

    (F.sub.T).sup.1/2 ·f.sub.1 /f.sub.T =0.843 and |β.sub.2W ·V.sup.1/2 |=0.973;

    (F.sub.T).sup.1/2 ·f.sub.1 /f.sub.T =0.846 and |β.sub.2W ·V.sup.1/2 |=0.949;

    (F.sub.T).sup.1/2 ·f.sub.1 /f.sub.T =0.883 and |β.sub.2W ·V.sup.1/2 |=1.000;

    (F.sub.T).sup.1/2 ·f.sub.1 /f.sub.T =0.888 and |β.sub.2W ·V.sup.1/2 |=1.002;

    (F.sub.T).sup.1/2 ·f.sub.1 /f.sub.T =0.895 and |β.sub.2W ·V.sup.1/2 |=0.998;

    (F.sub.T).sup.1/2 ·f.sub.1 /f.sub.T =0.900 and |β.sub.2W ·V.sup.1/2 |=0.998;

    (F.sub.T).sup.1/2 ·f.sub.1 /f.sub.T =0.881 and |β.sub.2W ·V.sup.1/2 |=1.006;

    (F.sub.T).sup.1/2 ·f.sub.1 /f.sub.T =0.880 and |β.sub.2W ·V.sup.1/2 |=1.010;

or

    (F.sub.T).sup.1/2 ·f.sub.1 /f.sub.T =0.839 and |β.sub.2W ·V.sup.1/2 |=0.954;

where f_(T) : the synthesized focal length of the entire system at thetelephoto end; F_(T) : the F-number at the telephoto end; f₁ : the focallength of said first lens unit; β_(2W) : the magnification of saidsecond lens unit at the wide-angle end; and V: the zoom ratio.
 3. A zoomlens according to claim 2, wherein said fifth lens unit includes atleast one aspherical surface.
 4. A zoom lens according to claim 2,wherein said zoom lens satisfies one of the following:

    (a) F.sub.T =3.59 f.sub.1 =8.601 f.sub.T =19.570 β.sub.2W =-0.215 V=19.57;

    (b) F.sub.T =3.65 f.sub.1 =8.968 f.sub.T =19.480 β.sub.2W =-0.237 V=19.48;

    (c) F.sub.T =3.59 f.sub.1 =8.848 f.sub.T =19.890 β.sub.2W =-0.218 V=19.89;

    (d) F.sub.T =3.59 f.sub.1 =8.813 f.sub.T =19.730 β.sub.2W =-0.214 V=19.73;

    (e) F.sub.T =3.69 f.sub.1 =9.142 f.sub.T =19.890 β.sub.2W =-0.224 V=19.89;

    (f) F.sub.T =3.69 f.sub.1 =9.184 f.sub.T =19.870 β.sub.2W =-0.226 V=19.87;

    (g) F.sub.T =3.69 f.sub.1 =9.261 f.sub.T =19.870 β.sub.2W =-0.224 V=19.87;

    (h) F.sub.T =3.69 f.sub.1 =9.318 f.sub.T =19.890 β.sub.2W =-0.224 V=19.89;

    (i) F.sub.T =3.69 f.sub.1 =9.109 f.sub.T =19.870 β.sub.2W =-0.226 V=19.87;

    (j) F.sub.T =3.69 f.sub.1 =9.107 f.sub.T =19.870 β.sub.2W =-0.227 V=19.87;

or

    (k) F.sub.T =2.84 f.sub.1 =9.811 f.sub.T =19.700 β.sub.2W =-0.215 V=19.70.


5. A zoom lens according to claim 4, wherein said fifth lens unitincludes at least one aspherical surface.
 6. A zoom lens comprising inthe following order from the object side:a first lens unit which isfixed in position during zooming and has a positive refractive power; asecond lens unit which is movable upon zooming and has a negativerefractive power; a third lens unit which is fixed in position duringzooming and has a positive refractive power; a fourth lens unit which ismovable upon zooming and has a negative refractive power; and a fifthlens unit consisting of a bonded lens which is fixed in position duringzooming and has a positive refractive power, an uninterrupted air gapextending from said fourth lens unit to said fifth lens unit, wherein,upon zooming from the wide-angle end to the telephoto end, said secondlens unit is unidirectionally moved from the object side to the imageside along an optical axis, and said fourth lens unit is moved along theoptical axis from the object side to the image side within a range fromthe wide-angle end to a middle position between the wide-angle andtelephoto ends, and is moved along the optical axis from the image sideto the object side within a range from the middle position to thetelephoto end.
 7. A zoom lens according to claim 6, wherein said zoomlens satisfies:

    (F.sub.T).sup.1/2 ·f.sub.1 /f.sub.T =0.900 and |β.sub.2W ·V.sup.1/2 |=0.998,

where f_(T) : the synthesized focal length of the entire system at thetelephoto end; F_(T) : the F-number at the telephoto end; f₁ : the focallength of said first lens unit; β_(2W) : the magnification of saidsecond lens unit at the wide-angle end; and V: the zoom ratio.
 8. A zoomlens according to claim 7, wherein said fifth lens unit includes atleast one aspherical surface.
 9. A zoom lens according to claim 6,wherein said fifth lens unit includes at least one aspherical surface.10. A zoom lens according to claim 6, wherein said zoom lens satisfies:

    F.sub.T =3.69 f.sub.1 =9.318 f.sub.T =19.890 β.sub.2W =-0.224 V=19.89,

where f_(T) : the synthesized focal length of the entire system at thetelephoto end; F_(T) : the F-number at the telephoto end; f₁ : the focallength of said first lens unit; β_(2W) : the magnification of saidsecond lens unit at the wide-angle end; and V: the zoom ratio.
 11. Azoom lens according to claim 10, wherein said fifth lens unit includesat least one aspherical surface.